Solution structure of human insulin-like growth factor II; recognition sites for receptors and binding proteins

Regulation and function of insulin and insulin-like growth factor receptor signalling

Receptors of insulin and insulin-like growth factors (IGFs) are receptor tyrosine kinases whose signalling controls multiple aspects of animal physiology throughout life. In addition to regulating metabolism and growth, insulin-IGF receptor signalling has recently been linked to a variety of new, cell type-specific functions. In the last century, key questions have focused on how structural differences of insulin and IGFs affect receptor activation, and how insulin-IGF receptor signalling translates into pleiotropic biological functions. Technological advances such as cryo-electron microscopy have provided a detailed understanding of how native and engineered ligands activate insulin-IGF receptors. In this Review, we highlight recent structural and functional insights into the activation of insulin-IGF receptors, and summarize new agonists and antagonists developed for intervening in the activation of insulin-IGF receptor signalling. Furthermore, we discuss recently identified regulatory mechanisms beyond ligand-receptor interactions and functions of insulin-IGF receptor signalling in diseases.

Activation of the insulin receptor by insulin-like growth factor 2

Insulin receptor (IR) controls growth and metabolism. Insulin-like growth factor 2 (IGF2) has different binding properties on two IR isoforms, mimicking insulin's function. However, the molecular mechanism underlying IGF2-induced IR activation remains unclear. Here, we present cryo-EM structures of full-length human long isoform IR (IR-B) in both the inactive and IGF2-bound active states, and short isoform IR (IR-A) in the IGF2-bound active state. Under saturated IGF2 concentrations, both the IR-A and IR-B adopt predominantly asymmetric conformations with two or three IGF2s bound at site-1 and site-2, which differs from that insulin saturated IR forms an exclusively T-shaped symmetric conformation. IGF2 exhibits a relatively weak binding to IR site-2 compared to insulin, making it less potent in promoting full IR activation. Cell-based experiments validated the functional importance of IGF2 binding to two distinct binding sites in optimal IR signaling and trafficking. In the inactive state, the C-terminus of α-CT of IR-B contacts FnIII-2 domain of the same protomer, hindering its threading into the C-loop of IGF2, thus reducing the association rate of IGF2 with IR-B. Collectively, our studies demonstrate the activation mechanism of IR by IGF2 and reveal the molecular basis underlying the different affinity of IGF2 to IR-A and IR-B.

Signaling Pathways of the Insulin-like Growth Factor Binding Proteins

The 6 high-affinity insulin-like growth factor binding proteins (IGFBPs) are multifunctional proteins that modulate cell signaling through multiple pathways. Their canonical function at the cellular level is to impede access of insulin-like growth factor (IGF)-1 and IGF-2 to their principal receptor IGF1R, but IGFBPs can also inhibit, or sometimes enhance, IGF1R signaling either through their own post-translational modifications, such as phosphorylation or limited proteolysis, or by their interactions with other regulatory proteins. Beyond the regulation of IGF1R activity, IGFBPs have been shown to modulate cell survival, migration, metabolism, and other functions through mechanisms that do not appear to involve the IGF-IGF1R system. This is achieved by interacting directly or functionally with integrins, transforming growth factor β family receptors, and other cell-surface proteins as well as intracellular ligands that are intermediates in a wide range of pathways. Within the nucleus, IGFBPs can regulate the diverse range of functions of class II nuclear hormone receptors and have roles in both cell senescence and DNA damage repair by the nonhomologous end-joining pathway, thus potentially modifying the efficacy of certain cancer therapeutics. They also modulate some immune functions and may have a role in autoimmune conditions such as rheumatoid arthritis. IGFBPs have been proposed as attractive therapeutic targets, but their ubiquity in the circulation and at the cellular level raises many challenges. By understanding the diversity of regulatory pathways with which IGFBPs interact, there may still be therapeutic opportunities based on modulation of IGFBP-dependent signaling.

Mutations at hypothetical binding site 2 in insulin and insulin-like growth factors 1 and 2

Elucidating how insulin and the related insulin-like growth factors 1 and 2 (IGF-1 and IGF-2) bind to their cellular receptors (IR and IGF-1R) and how the receptors are activated has been the holy grail for generations of scientists. However, deciphering the 3D structure of tyrosine kinase receptors and their hormone-bound complexes has been complicated by the flexible and dimeric nature of the receptors and the dynamic nature of their interaction with hormones. Therefore, mutagenesis of hormones and kinetic studies first became an important tool for studying receptor interactions. It was suggested that hormones could bind to receptors through two binding sites on the hormone surface called site 1 and site 2. A breakthrough in knowledge came with the solution of cryoelectron microscopy (cryoEM) structures of hormone-receptor complexes. In this chapter, we document in detail the mutagenesis of insulin, IGF-1, and IGF-2 with emphasis on modifications of the hypothetical binding site 2 in the hormones, and we discuss the results of structure-activity studies in light of recent cryoEM structures of hormone complexes with IR and IGF-1R.

The IGF system and longitudinal growth in preterm infants in relation to gestational age, birth weight and gender

OBJECTIVE

Growth factors in the blood of very preterm infants may reflect growth and contribute to the understanding of early development. We investigated postnatal levels of insulin-like growth factors (IGFs) in infants born very preterm and related them to early growth development.

DESIGN

Blood samples were analyzed weekly for IGF-I, IGF-II, IGF binding protein (BP)-1, IGFBP-3, and acid-label subunit (ALS).

METHODS

73 children born very preterm (gestational age (GA) <32 weeks) were divided according to their gender-specific birth weight standard deviation score (SDS) into either appropriate for GA (AGA) or small for GA (SGA). Fifty-two (71%) and forty-three (59%) infants completed follow-up with anthropometry at approximately 3 years and at 5 years of age respectively. Thirty-six subjects (49%) had blood sampling for IGF-I and IGFBP-3 measurements up to 3 years of age.

RESULTS

IGF-I, IGFBP-3, and ALS levels increased in all groups from week 31 to week 36, with generally lower levels in the SGAs, with a concomitant lower growth velocity. Postnatal ALS was strongly associated with IGF-I and IGFBP-3 in boys, girls and AGA infants. IGF-II was higher in earlier born preterms (GA < 27 weeks) at postmenstrual ages 27.5-29.9 weeks compared with SGAs and late GA (GA ≥ 27 weeks) preterms (p < .0001). IGF-II, in contrast to IGF-I, did not differ between SGAs and AGAs at weeks 31-36. Mean IGFBP-1 was highest in the SGAs compared to AGAs at mean week 28,5 and 31 (p = .001) and IGFBP-1 levels were elevated in relation to IGF-I in the SGAs at that period. At follow-up, the increase in IGF-I between week 31 and 33.5 was a significant positive determinant of height SDS at 3 and 5 years of age in forward multiple regression analysis, independent of target height.

CONCLUSION

This is the first study to investigate postnatal ALS levels in preterm infants. In very preterm infants, IGF-II is less affected by size at birth during early postnatal weeks compared with IGF-I. Early elevated IGFBP-1 might protect the SGA infants from an intense metabolic rate. Our results indicate that anabolic and metabolic processes during weeks 31-36 predicts later height.

IGF1 and IGF2 specificities to the two insulin receptor isoforms are determined by insulin receptor amino acid 718

Methods

Alanine scan of insulin receptor (IR)-B exon 11 and site-directed mutagenesis of amino acid 718 in human IR-A and IR-B were performed. Ligand affinities to wild type and mutated receptors were studied by displacement of radioactive insulin in binding assay on secreted soluble midi receptors or solubilized semi-purified full length receptors stably expressed in Baby Hamster Kidney cells. Phosphorylation of IR in response to insulin, IGF1 and IGF2 was measured using ELISA.

Results

Insulin, insulin detemir and insulin glargine maximally showed two fold differences in affinity for human IR-A and IR-B, but IGF1 and IGF2 had up to 10 fold preference for IR-A. Alanine scan of exon 11 revealed that position 718 is important for low IGF1 affinity to IR-B. Mutational analysis of amino acid residue 718 in IR-A and IR-B demonstrated that charge is important for IGF1 and IGF2 affinity but not important for insulin affinity. The affinity of IGF1 and IGF2 for the mutant IR-A P718K was comparable to the wild type IR-B whereas the affinity of IGF1 and IGF2 for the mutant IR-B K718P was comparable to the wild type IR-A. Changes in affinity were also reflected in the IR activation pattern.

Conclusion

Mutating position 718 in human IR-B to the proline found at position 718 in human IR-A increased IGF1 and IGF2 affinity to a level comparable to IR-A and mutating position 718 in IR-A to the lysine found at position 718 in IR-B decreased IGF1 and IGF2 affinity to a level comparable to IR-B, whereas a negatively charged glutamate did not. These changes in the affinities were also reflected in the IR phosphorylation pattern, meaning that position 718 is important for both affinity and activation of the receptor. It should be emphasized that none of the mutations affected insulin affinity, indicating that the mutations did not alter the overall receptor structure and that the effect is ligand specific.

Probing Receptor Specificity by Sampling the Conformational Space of the Insulin-like Growth Factor II C-domain

Insulin and insulin-like growth factors I and II are closely related protein hormones. Their distinct evolution has resulted in different yet overlapping biological functions with insulin becoming a key regulator of metabolism, whereas insulin-like growth factors (IGF)-I/II are major growth factors. Insulin and IGFs cross-bind with different affinities to closely related insulin receptor isoforms A and B (IR-A and IR-B) and insulin-like growth factor type I receptor (IGF-1R). Identification of structural determinants in IGFs and insulin that trigger their specific signaling pathways is of increasing importance in designing receptor-specific analogs with potential therapeutic applications. Here, we developed a straightforward protocol for production of recombinant IGF-II and prepared six IGF-II analogs with IGF-I-like mutations. All modified molecules exhibit significantly reduced affinity toward IR-A, particularly the analogs with a Pro-Gln insertion in the C-domain. Moreover, one of the analogs has enhanced binding affinity for IGF-1R due to a synergistic effect of the Pro-Gln insertion and S29N point mutation. Consequently, this analog has almost a 10-fold higher IGF-1R/IR-A binding specificity in comparison with native IGF-II. The established IGF-II purification protocol allowed for cost-effective isotope labeling required for a detailed NMR structural characterization of IGF-II analogs that revealed a link between the altered binding behavior of selected analogs and conformational rearrangement of their C-domains.

An exon splice enhancer primes IGF2:IGF2R binding site structure and function evolution

Placental development and genomic imprinting coevolved with parental conflict over resource distribution to mammalian offspring. The imprinted genes IGF2 and IGF2R code for the growth promoter insulin-like growth factor 2 (IGF2) and its inhibitor, mannose 6-phosphate (M6P)/IGF2 receptor (IGF2R), respectively. M6P/IGF2R of birds and fish do not recognize IGF2. In monotremes, which lack imprinting, IGF2 specifically bound M6P/IGF2R via a hydrophobic CD loop. We show that the DNA coding the CD loop in monotremes functions as an exon splice enhancer (ESE) and that structural evolution of binding site loops (AB, HI, FG) improved therian IGF2 affinity. We propose that ESE evolution led to the fortuitous acquisition of IGF2 binding by M6P/IGF2R that drew IGF2R into parental conflict; subsequent imprinting may then have accelerated affinity maturation.

Molecular basis for the recognition and cleavages of IGF-II, TGF-alpha, and amylin by human insulin-degrading enzyme

Insulin-degrading enzyme (IDE) is involved in the clearance of many bioactive peptide substrates, including insulin and amyloid-beta, peptides vital to the development of diabetes and Alzheimer's disease, respectively. IDE can also rapidly degrade hormones that are held together by intramolecular disulfide bond(s) without their reduction. Furthermore, IDE exhibits a remarkable ability to preferentially degrade structurally similar peptides such as the selective degradation of insulin-like growth factor (IGF)-II and transforming growth factor-alpha (TGF-alpha) over IGF-I and epidermal growth factor, respectively. Here, we used high-accuracy mass spectrometry to identify the cleavage sites of human IGF-II, TGF-alpha, amylin, reduced amylin, and amyloid-beta by human IDE. We also determined the structures of human IDE-IGF-II and IDE-TGF-alpha at 2.3 A and IDE-amylin at 2.9 A. We found that IDE cleaves its substrates at multiple sites in a biased stochastic manner. Furthermore, the presence of a disulfide bond in amylin allows IDE to cut at an additional site in the middle of the peptide (amino acids 18-19). Our amylin-bound IDE structure offers insight into how the structural constraint from a disulfide bond in amylin can alter IDE cleavage sites. Together with NMR structures of amylin and the IGF and epidermal growth factor families, our work also reveals the structural basis of how the high dipole moment of substrates complements the charge distribution of the IDE catalytic chamber for the substrate selectivity. In addition, we show how the ability of substrates to properly anchor their N-terminus to the exosite of IDE and undergo a conformational switch upon binding to the catalytic chamber of IDE can also contribute to the selective degradation of structurally related growth factors.

Insulin-like growth factor-I (IGF-I): solution properties and NMR chemical shift assignments near physiological pH

OBJECTIVE

Insulin-like growth factor-I (IGF-I) plays important roles in normal growth and development, as well as in disease states, and its structure and function have been studied extensively using nuclear magnetic resonance (NMR) spectroscopy. However, IGF-I typically gives poor quality NMR spectra containing many broad peaks, because of aggregation at the protein concentrations generally required for NMR experiments as well as the internal dynamics of the molecule. The present study was undertaken to determine a reliable set of assignments under more physiological conditions.

DESIGN

Several reports of chemical shift assignments have been published previously for IGF-I either bound to a ligand or at relatively low pH (approximately 3-4), but there are many contradictions among them, reflecting the poor behaviour of IGF-I. Low pH conditions are also suboptimal for the analysis of interactions between IGF-I and IGF binding proteins (IGFBP) or IGFBP fragments. Spectra were recorded at low concentrations in order to identify conditions of temperature and pH where all peaks could be observed.

RESULTS

We show that good quality 2D (1)H-(15)N HSQC spectra of (15)N-labelled IGF-I can be obtained at pH 6 and 37 degrees C, much closer to physiological conditions, by using lower IGF-I concentrations (0.05 mM). Surprisingly, at this concentration and temperature, spectra were of better quality at pH 6 than at pH 4, in contrast to previous observations made at millimolar concentrations of IGF-I. We were then also able to assign the chemical shifts of IGF-I at pH 6 and 37 degrees C using 3D heteronuclear spectra recorded on a 0.7 mM (15)N/(13)C-labelled IGF-I sample.

CONCLUSION

These results provide a valuable resource for future studies of the structure, dynamics, folding, and binding interactions of IGF-I, as well as analogues thereof, by means of NMR spectroscopy.

A novel approach to identify two distinct receptor binding surfaces of insulin-like growth factor II

Very little is known about the residues important for the interaction of insulin-like growth factor II (IGF-II) with the type 1 IGF receptor (IGF-1R) and the insulin receptor (IR). Insulin, to which IGF-II is homologous, is proposed to cross-link opposite halves of the IR dimer through two receptor binding surfaces, site 1 and site 2. In the present study we have analyzed the contribution of IGF-II residues equivalent to insulin's two binding surfaces toward the interaction of IGF-II with the IGF-1R and IR. Four "site 1" and six "site 2" analogues were produced and analyzed in terms of IGF-1R and IR binding and activation. The results show that Val(43), Phe(28), and Val(14) (equivalent to site 1) are critical to IGF-1R and IR binding, whereas mutation to alanine of Gln(18) affects only IGF-1R and not IR binding. Alanine substitutions at Glu(12), Asp(15), Phe(19), Leu(53), and Glu(57) analogues resulted in significant (>2-fold) decreases in affinity for both the IGF-1R and IR. Furthermore, taking a novel approach using a monomeric, single-chain minimized IGF-1R we have defined a distinct second binding surface formed by Glu(12), Phe(19), Leu(53), and Glu(57) that potentially engages the IGF-1R at one or more of the FnIII domains.

The human insulin superfamily of polypeptide hormones

The identification in the 1950s of insulin, an essential carbohydrate regulatory hormone, as consisting of not one but two peptide chains linked by three disulfide bonds in a distinctive pattern was a milestone in peptide chemistry. When it was later found that relaxin also possessed a similar overall structure, the term 'insulin superfamily' was coined. Use of methods of conventional protein chemistry followed by recombinant DNA and more recently bioinformatics has led to the recognition that insulin is the precursor to a large protein superfamily that extends beyond the human. Insulin-like peptides are found not only in vertebrates such as mammals, birds, reptiles, amphibians but also in the invertebrates such as chordates, molluscs and insects. All superfamily members share the distinctive insulin structural motif. In the human, there exists ten members of the superfamily, each of which are expressed on the ribosome as a single-chain pre-prohormone that undergoes proteolytic processing to produce eight double-chain mature proteins and two single-chain forms. The six cysteine residues that form the three insulin disulfide cross-links - one intramolecular within the A-chain and two intermolecular between that A- and B-chains - are absolutely conserved across all members of the superfamily. They are responsible for imparting a similar overall tertiary structure. The human insulin superfamily members have each evolved to assume remarkably distinctive biological functions ranging from glucose homeostasis to neuroendocrine actions. That such diversity is contained within a modestly sized superfamily is testament to efficiency of the insulin structural motif as an evolutionary template.

IGF-I binding to the IGF-I receptor is affected by contaminants in commercial BSA: the contaminants are proteins with IGF-I binding properties

OBJECTIVE

To determine whether different albumins have an effect on IGF-I binding assays.

METHODS

We have studied the effect of five different albumins in plate antibody capture binding assay. For IGF-IR studies the IGF-IR specific antibody 24-31 was used and for IR/IGF-IR hybrid receptors the IR specific antibody 83-7 was used. Binding to IGF-IR was studied by displacement of (125)I-IGF-I with IGF-I in the absence or presence of 0.1%, 0.5% or 1% (w/v) albumin. The IR/IGF-IR hybrid receptors were studied in the presence of 0.5% (w/v) of HSA A-1887 and BSA A-7888 and with IGF-I or insulin displacement of (125)I-IGF-I. The albumins used were purchased from Sigma-Aldrich. Two batches of albumins from each catalog number were tested. The albumins were: HSA A-1887, BSA A-4503, BSA A-6003, BSA A-7030, and BSA A-7888. Contaminants in the albumins were characterized as proteins with IGF-I binding properties by cross-linking to (125)I-IGF-I and SDS-page analysis.

RESULTS

BSA A-4503, A-7030 and A-7888 from Sigma-Aldrich contain proteins with IGF-I binding properties. These contaminants increased the determined EC50 for displacement of (125)I-IGF-I from IGF-IR up to 40-fold in a BSA dependent manner. The presence of BSA-7888 in binding experiments increased the determined EC50 for IR/IGF-IR hybrid receptors 8-16-fold.

CONCLUSIONS

When IGF-I is characterized with respect to the effect on living cells and on binding to potential receptors unspecific binding to surfaces is often prevented by the addition of albumin in the assay. Here we report that when binding to the classical IGF-IR and IR/IGF-IR hybrid receptors are studied the measured EC50 values can be albumin dependent if it is contaminated with proteins with IGF-I binding properties. The free IGF-I concentration will be lower than estimated. Thus, the contaminated BSA preparations result in artifacts leading to misinterpretations and underestimation of the effect of IGF-I. Our results provide one possible explanation as to why different laboratories report different EC50 values for IGF-I.

Cooperativity of the N- and C-terminal domains of insulin-like growth factor (IGF) binding protein 2 in IGF binding

A family of six insulin-like growth factor (IGF) binding proteins (IGFBP-1-6) binds IGF-I and IGF-II with high affinity and thus regulates their bioavailability and biological functions. IGFBPs consist of N- and C-terminal domains, which are highly conserved and cysteine-rich, joined by a variable linker domain. The role of the C-domain in IGF binding is not completely understood in that C-domain fragments have very low or even undetectable IGF binding affinity, but loss of the C-domain dramatically disrupts IGF binding by IGFBPs. We recently reported the solution structure and backbone dynamics of the C-domain of IGFBP-2 (C-BP-2) and identified a pH-dependent heparin binding site [Kuang, Z., Yao, S., Keizer, D. W., Wang, C. C., Bach, L. A., Forbes, B. E., Wallace, J. C., and Norton, R. S. (2006) Structure, dynamics and heparin binding of the C-terminal domain of insulin-like growth factor-binding protein-2 (IGFBP-2), J. Mol. Biol. 364, 690-704]. Here, we have analyzed the molecular interactions among the N-domain of IGFBP-2 (N-BP-2), C-BP-2, and IGFs using cross-linking and nuclear magnetic resonance (NMR) spectroscopy. The binding of C-BP-2 to the IGF-I.N-BP-2 binary complex was significantly stronger than the binding of C-BP-2 to IGF-I alone, switching from intermediate exchange to slow exchange on the NMR time scale. A conformational change or stabilization of the IGF-I Phe49-Leu54 region and the Phe49 aromatic ring upon binding to the N-domains, as well as an interdomain interaction between N-BP-2 and C-BP-2 (which is also detectable in the absence of ligand), may contribute to this cooperativity in IGF binding. Glycosaminoglycan binding by IGFBPs can affect their IGF binding although the effects appear to differ among different IGFBPs; here, we found that heparin bound to the IGF-I.N-BP-2.C-BP-2 ternary complex, but did not cause it to dissociate.

Transcriptional regulation and biological significance of the insulin like growth factor II gene

The insulin like growth factors I and II are the most ubiquitous in the mammalian embryo. Moreover they play a pivotal role in the development and growth of tumours. The bioavailability of these growth factors is regulated on a transcriptional as well as on a posttranslational level. The expression of non-signalling receptors as well as binding proteins does further tune the local concentration of IGFs. This paper aims at reviewing how the transcription of the IGF genes is regulated. The biological significance of these control mechanisms will be discussed.

Kinetics of Insulin-like Growth Factor II (IGF-II) Interaction with Domain 11 of the Human IGF-II/Mannose 6-phosphate Receptor: Function of CD and AB Loop Solvent-exposed Residues

Ligands of the IGF-II/mannose 6-phosphate receptor (IGF2R) include IGF-II and mannose 6-phosphate modified proteins. Disruption of the negative regulatory effects of IGF2R on IGF-II-induced growth can lead to embryonic lethality and cancer promotion. Of the 15 IGF2R extracellular domains, domains 1-3 and 11 are known to have a conserved beta-barrel structure similar to that of avidin and the cation-dependent mannose 6-phosphate receptor, yet only domain 11 binds IGF-II with high specificity and affinity. In order to define the functional basis of this critical biological interaction, we performed alanine mutagenesis of structurally determined solvent-exposed loop residues of the IGF-II-binding site of human domain 11, expressed these mutant forms in Pichia pastoris, and determined binding kinetics with human IGF-II using isothermal calorimetry and surface plasmon resonance with transition state thermodynamics. Two hydrophobic residues in the CD loop (F1567 and I1572) were essential for binding, with a further non-hydrophobic residue (T1570) that slows the dissociation rate. Aside from alanine mutations of AB loop residues that decrease affinity by modifying dissociation rates (e.g. Y1542), a novel mutation (E1544A) of the AB loop enhanced affinity by threefold compared to wild-type. Conversion from an acidic to a basic residue at this site (E1544K) results in a sixfold enhancement of affinity via modification principally of the association rate, with enhanced salt-dependence, decreased entropic barrier and retained specificity. These data suggest that a functional hydrophobic binding site core is formed by I1572 and F1567 located in the CD loop, which initially anchors IGF-II. Within the AB loop, residues normally act to either stabilise or function as negative regulators of the interaction. These findings have implications for the molecular architecture and evolution of the domain 11 IGF-II-binding site, and the potential interactions with other domains of IGF2R.

Molecular interactions of the IGF system

The insulin-like growth factor (IGF) system is a complex network of two soluble ligands; several cell surface transmembrane receptors and six soluble high-affinity binding-proteins. The IGF system is essential for normal embryonic and postnatal growth, and plays an important role in the function of a healthy immune system, lymphopoiesis, myogenesis and bone growth among other physiological functions. Deregulation of the IGF system leads to stimulation of cancer cell growth and survival. In order to manipulate the IGF system in the treatment of certain disorders, we must understand the protein-protein interactions at a molecular level. The complex molecular interactions of the ligands and receptors of the IGF system underlie all the biological actions mentioned above and will be the focus of this review.

Identification of Selective Basophil Chemoattractants in Human Nasal Polyps as Insulin-Like Growth Factor-1 and Insulin-Like Growth Factor-2

In a search for novel leukocyte chemoattractants at sites of allergic inflammation, we found basophil-selective chemoattractant activity in extracts of human nasal polyps. The extracts were fractionated by reverse phase HPLC, and the resulting fractions were tested for leukocyte-stimulating activity using sensitive shape change assays. The basophil-selective activity detected was not depleted by a poxvirus CC-chemokine-binding protein affinity column. This activity was further purified by HPLC, and proteins in the bioactive fractions were analyzed by tandem electrospray mass spectrometry. Insulin-like growth factor-2 (IGF-2) was identified in these HPLC fractions, and the basophil-stimulating activity was inhibited by an anti-IGF-2-neutralizing Ab. Recombinant IGF-2 induced a substantial shape change response in basophils, but not eosinophils, neutrophils, or monocytes. IGF-2 stimulated chemokinesis of basophils, but not eosinophils or neutrophils, and synergized with eotaxin-1/CCL11 in basophil chemotaxis. IGF-2 also caused up-regulation of basophil CD11b expression and inhibited apoptosis, but did not stimulate degranulation or Ca(2+) flux. Recombinant IGF-1 exhibited similar basophil-selective effects as IGF-2, and both growth factors were detected in nasal polyp extracts by ELISA. This is the first demonstration of chemokinetic factors that increase the motility of basophils, but do not act on other granulocytes or monocytes. IGF-1 and IGF-2 could play a role in the selective recruitment of basophils in vivo.

Role of N- and C-terminal residues of insulin-like growth factor (IGF)-binding protein-3 in regulating IGF complex formation and receptor activation

Insulin-like growth factor-binding protein-3 (IGFBP-3), the major IGFBP in the circulation, sequesters IGF in a stable ternary complex with the acid-labile subunit. The high affinity IGF-binding site is proposed to reside within an N-terminal hydrophobic domain in IGFBP-3, but C-terminal residues have also been implicated in the homologous protein IGFBP-5. We have mutated in various combinations Leu(77), Leu(80), and Leu(81) in the N terminus and Gly(217) and Gln(223) in the C terminus of IGF-BP-3. All mutants retained immunoreactivity toward a polyclonal IGFBP-3 antibody, whereas IGF ligand blotting showed that all of the mutants had reduced binding to IGFs. Both solution IGF binding assays and BIAcore analysis indicated that mutations to the N-terminal region caused greater reduction in IGF binding activity than C-terminal mutations. The combined N- and C-terminal mutants showed undetectable binding to IGF-I but retained <10% IGF-II binding activity. Reduced ternary complex formation was seen only in mutants that had considerably reduced IGF-I binding, consistent with previous studies indicating that the binary IGF.IGFBP-3 complex is required for acid-labile subunit binding. Decreased IGF binding was also reflected in the inability of the mutants to inhibit IGF-I signaling in IGF receptor overexpressing cells. However, when present in excess, IGFBP-3 analogs defined as non-IGF-binding by biochemical assays could still inhibit IGF signaling. This suggests that residual binding activity of IGFBP-3 mutants may still be sufficient to inhibit IGF biological activity and questions the use of such analogs to study IGF-independent effects of IGFBP-3.

Structural determinants for high-affinity binding of insulin-like growth factor II to insulin receptor (IR)-A, the exon 11 minus isoform of the IR

The insulin receptor (IR) lacking the alternatively spliced exon 11 (IR-A) is preferentially expressed in fetal and cancer cells. The IR-A has been identified as a high-affinity receptor for insulin and IGF-II but not IGF-I, which it binds with substantially lower affinity. Several cancer cell types that express the IR-A also overexpress IGF-II, suggesting a possible autocrine proliferative loop. To determine the regions of IGF-I and IGF-II responsible for this differential affinity, chimeras were made where the C and D domains were exchanged between IGF-I and IGF-II either singly or together. The abilities of these chimeras to bind to, and activate, the IR-A were investigated. We also investigated the ability of these chimeras to bind and activate the IR exon 11+ isoform (IR-B) and as a positive control, the IGF-I receptor (IGF-1R). We show that the C domain and, to a lesser extent, the D domains represent the principal determinants of the binding differences between IGF-I and IGF-II to IR-A. The C and D domains of IGF-II promote higher affinity binding to the IR-A than the equivalent domains of IGF-I, resulting in an affinity close to that of insulin for the IR-A. The C and D domains also regulate the IR-B binding specificity of the IGFs in a similar manner, although the level of binding for all IGF ligands to IR-B is lower than to IR-A. In contrast, the C and D domains of IGF-I allow higher affinity binding to the IGF-1R than the analogous domains of IGF-II. Activation of IGF-1R by the chimeras reflected their binding affinities whereas the phosphorylation of the two IR isoforms was more complex.

Glycosylation-independent targeting enhances enzyme delivery to lysosomes and decreases storage in mucopolysaccharidosis type VII mice

Enzyme-replacement therapy is an established means of treating lysosomal storage diseases. Infused therapeutic enzymes are targeted to lysosomes of affected cells by interactions with cell-surface receptors that recognize carbohydrate moieties, such as mannose and mannose 6-phosphate, on the enzymes. We have tested an alternative, peptide-based targeting system for delivery of enzymes to lysosomes in a murine mucopolysaccharidosis type VII (MPS VII) model. This strategy depends on the interaction of a fragment of insulin-like growth factor II (IGF-II), with the IGF-II binding site on the bifunctional, IGF-II cation-independent mannose 6-phosphate receptor. A chimeric protein containing a portion of mature human IGF-II fused to the C terminus of human beta-glucuronidase was taken up by MPS VII fibroblasts in a mannose 6-phosphate-independent manner, and its uptake was inhibited by the addition of IGF-II. Furthermore, the tagged enzyme was delivered effectively to clinically significant tissues in MPS VII mice and was effective in reversing the storage pathology. The tagged enzyme was able to reduce storage in glomerular podocytes and osteoblasts at a dose at which untagged enzyme was much less effective. This peptide-based, glycosylation-independent lysosomal targeting system may enhance enzyme-replacement therapy for certain human lysosomal storage diseases.

Quinoline-carboxylic acids are potent inhibitors that inhibit the binding of insulin-like growth factor (IGF) to IGF-binding proteins

4-benzylquinolines 5, based on a series of isoquinolines 1, were prepared and tested as inhibitors of the IGF/IGFBP-3 complex based on their ability to displace IGF-I from its binding to IGF-binding protein-3. SAR studies on the 6,7-dihydroxy moiety of the quinoline 5a showed that the catecol moiety could be replaced with other functional groups. Computational modeling of the 5a/mini-IGFBP-5 complex revealed the possible binding site of 5a on IGFBP-5.

The different folding behavior of insulin and insulin-like growth factor 1 is mainly controlled by their B-chain/domain

Although insulin and insulin-like growth factor 1 (IGF-1) share homologous sequence, similar tertiary structure, weakly overlapped biological activity, and a common ancestor, the two highly homologous sequences encode different folding behavior: insulin folds into one unique stable tertiary structure while IGF-1 folds into two disulfide isomers with similar thermodynamic stability. To further elucidate the molecular mechanism of their different folding behavior, we prepared two single-chain hybrids of insulin and IGF-1, Ins(A)/IGF-1(B) and Ins(B)/IGF-1(A), as well as a mini-IGF-1 by means of protein engineering and studied their structure as well as folding behavior. Both mini-IGF-1 and Ins(A)/IGF-1(B) fold into two thermodynamically stable disulfide isomers in vivo and in vitro just like that of IGF-1, while Ins(B)/IGF-1(A) folds into one unique thermodynamically stable tertiary structure in vivo and in vitro just like that of insulin. So we deduce that the different folding behavior of insulin and IGF-1 is mainly controlled by their B-chain/domain. By V8 endoproteinase digestion and circular dichroism analysis, as well as insulin receptor binding assay, we deduce that Ins(B)/IGF-1(A), isomer 2 of mini-IGF-1, and isomer 2 of Ins(A)/IGF-1(B) adopt native IGF-1/insulin-like three-dimensional structure with native disulfides, while isomer 1 of mini-IGF-1 and isomer 1 of Ins(A)/IGF-1(B) adopt the swap IGF-1-like three-dimensional structure with swap disulfides.

Mutation of three critical amino acids of the N-terminal domain of IGF-binding protein-3 essential for high affinity IGF binding

The N-terminal domain is conserved in all members of the IGF-binding protein superfamily. Most recently, studies have demonstrated the importance of an IGF-binding protein N-terminal hydrophobic pocket for IGF binding. To examine more critically the amino acids important for IGF binding within the full-length IGF-binding protein-3 protein while minimizing changes in the tertiary structure, we targeted residues I56, L80, and L81 within the proposed hydrophobic pocket for mutation. With a single change at these sites to the nonconserved glycine there was a notable decrease in binding. A greater reduction was seen when both L80 and L81 were substituted with glycine, and complete loss of affinity for IGF-I and IGF-II occurred when all three targeted amino acids were changed to glycine. Furthermore, the ability of the IGF-binding protein-3 mutants to inhibit IGF-I-stimulated phosphorylation of its receptor was a reflection of their affinity for IGF, with the lowest affinity mutants having the least inhibitory effect. These studies, thus, support the hypothesis that an N-terminal hydrophobic pocket is the primary site of high affinity binding of IGF to IGF-binding protein-3. The mutants provide a tool for future studies directed at IGF-dependent and IGF-independent actions of IGF-binding protein-3.

NMR 15N relaxation of the insulin-like growth factor (IGF)-binding domain of IGF binding protein-5 (IGFBP-5) determined free in solution and in complex with IGF-II

15N NMR relaxation rates of mini-IGFBP-5, an N-terminal insulin-like growth factor binding domain of the insulin-like growth factor binding protein 5 (IGFBP-5), were analysed at three field strengths using the Lipari-Szabo procedure (see below) and reduced spectral density methods. Isotropic and anisotropic Lipari-Szabo models were analysed and an analytical formula for the overall correlation time for anisotropic molecules is presented. Mini-IGFBP-5 was found to be mainly rigid on fast ps time scales except for 11 unstructured flexible residues at the C-terminus. The insulin-like growth factor binding loop in the apo-protein exhibits small amounts of flexibility on fast time scales (ps to ns) but several loop residues show significant exchange broadening. These loop residues display no exchange broadening in the complex of IGF-II/mini-IGFBP-5. The isotropic overall tumbling time in solution at 31 degrees C of mini-IGFBP-5 complexed to IGF-II is tauc = 18.4 +/- 0.2 ns indicating a strong tendency for aggregation.

Substitutions for hydrophobic amino acids in the N-terminal domains of IGFBP-3 and -5 markedly reduce IGF-I binding and alter their biologic actions

Insulin-like growth factor-binding protein-3 and -5 (IGFBP-3 and -5) have been shown to bind insulin-like growth factor-I and -II (IGF-I and -II) with high affinity. Previous studies have proposed that the N-terminal region of IGFBP-5 contains a hydrophobic patch between residues 49 and 74 that is required for high affinity binding. These studies were undertaken to determine if mutagenesis of several of these residues resulted in a reduction of the affinity of IGFBP-3 and -5 for IGF-I. Substitutions for residues 68, 69, 70, 73, and 74 in IGFBP-5 (changing one charged residue, Lys(68), to a neutral one and the four hydrophobic residues to nonhydrophobic residues) resulted in an approximately 1000-fold reduction in the affinity of IGFBP-5 for IGF-I. Substitutions for homologous residues in IGFBP-3 also resulted in a >1000-fold reduction in affinity. The physiologic consequence of this reduction was that IGFBP-3 and -5 became very weak inhibitors of IGF-I-stimulated cell migration and DNA synthesis. Likewise, the ability of IGFBP-5 to inhibit IGF-I-stimulated receptor phosphorylation was attenuated. These changes did not appear to be because of alterations in protein folding induced by mutagenesis, because the IGFBP-5 mutant was fully susceptible to proteolytic cleavage by a specific IGFBP-5 protease. In summary, residues 68, 69, 70, 73, and 74 in IGFBP-5 appear to be critical for high affinity binding to IGF-I. Homologous residues in IGFBP-3 are also required, suggesting that they form a similar binding pocket and that for both proteins these residues form an important component of the core binding site. The availability of these mutants will make it possible to determine if there are direct, non-IGF-I-dependent effects of IGFBP-3 and -5 on cellular physiologic processes in cell types that secrete IGF-I.

Insulin-like growth factor (IGF)-binding proteins: interactions with IGFs and intrinsic bioactivities

The insulin-like growth factor (IGF)-binding proteins (IGFBPs) are a family of six homologous proteins with high binding affinity for IGF-I and IGF-II. Information from NMR and mutagenesis studies is advancing knowledge of the key residues involved in these interactions. IGF binding may be modulated by IGFBP modifications, such as phosphorylation and proteolysis, and by cell or matrix association of the IGFBPs. All six IGFBPs have been shown to inhibit IGF action, but stimulatory effects have also been established for IGFBP-1, -3, and -5. These generally involve a decrease in IGFBP affinity and may require cell association of the IGFBP, but precise mechanisms are unknown. The same three IGFBPs have well established effects that are independent of type I IGF receptor signaling. IGFBP-1 exerts these effects by signaling through alpha(5)beta(1)-integrin, whereas IGFBP-3 and -5 may have specific cell-surface receptors with serine kinase activity. The regulation of cell sensitivity to inhibitory IGFBP signaling may play a role in the growth control of malignant cells.

Solution structure and backbone dynamics of long-[Arg(3)]insulin-like growth factor-I

Long-[Arg(3)]insulin-like growth factor-I (IGF-I) is a potent analog of insulin-like growth factor-I that has been modified by a Glu(3) --> Arg mutation and a 13-amino acid extension appended to the N terminus. We have determined the solution structure of (15)N-labeled Long-[Arg(3)]-IGF-I using high resolution NMR and restrained molecular dynamics techniques to a precision of 0.82 +/- 0.28 A root mean square deviation for the backbone heavy atoms in the three alpha-helices and 3.5 +/- 0.9 A root mean square deviation for all backbone heavy atoms excluding the 8 N-terminal residues and the 8 C-terminal eight residues. Overall, the structure of the IGF-I domain is consistent with earlier studies of IGF-I with some minor changes remote from the N terminus. The major variations in the structure, compared with IGF-I, occur at the N terminus with a substantial reorientation of the N-terminal three residues of the IGF-I domain. These results are interpreted in terms of the lower binding affinity for insulin-like growth factor-binding proteins. The backbone dynamics of Long-[Arg(3)]IGF-I were investigated using (15)N nuclear spin relaxation and the heteronuclear nuclear Overhauser enhancement (NOE). There is a considerable degree of flexibility in Long-[Arg(3)]IGF-I, even in the alpha-helices, as indicated by an average ((1)H)(15)N NOE of 0.55 for the regions. The largest heteronuclear NOEs are observed in the helical regions, lower heteronuclear NOEs are observed in the C-domain loop separating helix 1 from helix 2, and negative heteronuclear NOEs are observed in the N-terminal extension and at the C terminus. Despite these data indicating conformational flexibility for the N-terminal extension, slow amide proton exchange was observed for some residues in this region, suggesting some transitory structure does exist, possibly a molten helix. A certain degree of flexibility may be necessary in all insulin-like growth factors to enable association with various receptors and binding proteins.

Total alanine-scanning mutagenesis of insulin-like growth factor I (IGF-I) identifies differential binding epitopes for IGFBP-1 and IGFBP-3

The bioavailability of insulin-like growth factor I (IGF-I) in the serum and tissues is controlled by members of the IGF binding protein family (IGFBP). These proteins form high-affinity complexes with IGF-I and thereby either inhibit or potentiate its mitogenic and metabolic effects. Thus, understanding the IGF-IGFBP interaction at the molecular level is crucial for attempts to modulate IGF-I activity in vivo. We have systematically investigated the binding contribution of each IGF-I amino acid side chain toward IGFBP-1 and IGFBP-3, combining alanine-scanning mutagenesis and monovalent phage display. Surprisingly, most IGF-I residues could be substituted by alanines, resulting in less than 5-fold affinity losses for IGFBP-3. In contrast, binding of IGFBP-1 was more sensitive to alanine substitutions in IGF-I. The glutamate and phenylalanine at positions 3 and 49 were identified as major specificity determinants for IGFBP-1: the corresponding alanine mutations, E3A and F49A, selectively decreased IGFBP-1 binding by 34- and 100-fold, whereas IGFBP-3 affinity was not affected or reduced maximally 4-fold. No side chain specificity determinant was found for IGFBP-3. Instead, our results suggest that the N-terminal backbone region of IGF-I is important for binding to IGFBP-3. The fact that the functional binding epitopes on IGF-I are overlapping but distinct for both binding proteins may be exploited to design binding protein-specific IGF variants.

The insulin-like growth factor (IGF)binding protein 1 binding epitope on IGF-I probed by heteronuclear NMR spectroscopy and mutational analysis

NMR spectroscopy studies and biosensor interaction analysis of native and site-directed mutants of insulin-like growth factor I (IGF-I) was applied to identify the involvement of individual residues in IGF-I binding to IGF-binding protein 1 (IGFBP-1). Backbone NMR chemical shifts were found to be affected by IGFBP-1 binding in the following residues: Pro2, Glu3, Cys6, Gly7, Gly19, Pro28-Gly30, Gly32, Arg36, Arg37, Gln40-Gly42, Pro63, Lys65, Pro66, and Lys68-Ala70. Three IGF-I arginine side chains were identified by NMR to participate in IGFBP-1 binding. All IGF-I arginine residues were replaced by alanines, using site-directed mutagenesis, in four single substituted variants, IGF-I(R21A), IGF-I(R50A), IGF-I(R55A), and IGF-I(R56A), and one double replacement mutant, IGF-I(R36A/R37A). Biosensor interaction analysis binding studies demonstrate the involvement of Arg36-Arg37 and Arg50 in IGFBP-1 binding, while experiments with the IGF-I receptor implicate Arg21, Arg36-Arg37, and Arg56 as part of the receptor binding epitope. These overlapping binding surfaces explain why IGF-I receptor and IGFBP-1 binding to IGF-I is competitive. The C terminus of free, but not IGFBP-1-bound, IGF-I is found to exist in two distinct, NMR-detectable conformations at 30 degreesC. One possible explanation for this structural heterogeneity could be cis-trans isomerization of the Cys6-Cys48 disulfide bond.

Secretion in Escherichia coli and phage-display of recombinant insulin-like growth factor binding protein-2

Insulin-like growth factors (IGFs) promote cell growth and differentiation. Their actions are regulated by six different, but related, binding proteins (IGFBPs). To investigate the molecular interactions between IGFs and IGFBPs, an Escherichia coli based production method and a phage display system has been developed. The cDNA for bovine IGFBP-2 was inserted between regions coding for the pelB signal sequence and geneIII product, g3p, of bacteriophage fd in a phagemid vector to generate pGF14. The coding sequences of IGFBP-2 and g3p were separated by an amber stop codon and a flexible linker containing the cleavage recognition site for H64A subtilisin. Using this system in BL21, a non-supE strain lacking ompT, most product, approximately 4 mg 1(-1) of IGFBP-2, was obtained in the growth medium. The bacterially derived IGFBP-2 had a correct N-terminal sequence, molecular mass on SDS-PAGE and the same affinity for IGF-1 and IGF-II as IGFBP-2 from mammalian cells. In a supE strain of E. coli, IGFBP-2 was produced as an IGF-binding fusion to g3p. Procedures for display and approximately 10000 fold enrichment of IGFBP-2 bearing phage using adsorption to IGF-II coated microtitre plates were developed. Thus IGFBP-2 can be secreted in E. coli and displayed on filamentous phage. These can be selectively enriched by binding to immobilised IGF-II.

Binding sites and binding properties of binary and ternary complexes of insulin-like growth factor-II (IGF-II), IGF-binding protein-3, and acid-labile subunit

We have examined regions of rat IGF-binding protein-3 (IGFBP-3) important for complex formations using two kinds of deletion mutants, three kinds of chimera molecules between rat IGFBP-3 and rat IGFBP-2, and a synthetic peptide (41 residues, Glu52-Ala92) derived from rat IGFBP-3. Solid-phase binding assays using 96-well microtiter plates were designed to quantitate the relative binding affinities. It was found that not only the IGFBP-3 derivatives with the amino-terminal, cysteine-rich domain (N domain) but also the synthetic peptide maintained affinity for IGF-II. Ternary complex formation was observed with full-length IGFBP-3 and chimera IGFBP, the carboxyl-terminal cysteine-rich domain (C domain) of which was derived from IGFBP-3, unlike the mutants lacking the C domain and the chimera IGFBPs, the C domain of which was derived from IGFBP-2. These results were confirmed by affinity cross-linking experiments. Furthermore, the IGFBP-3 derivatives that possessed the C domain of IGFBP-3 bound to the acid-labile subunit, even in the absence of IGFs. Finally, we observed sites in IGF-II important for the ternary complex formation using various IGF-II mutants. These IGF-II mutants, which contained a substitution of Tyr27 for Leu, had extremely reduced activity. These results strongly suggest that: 1) the N domain, containing at least Glu52-Ala92, of rat IGFBP-3 is important for binding to IGF-II; 2) the C domain of IGFBP-3 is essential for binding to the acid-labile subunit both in the presence and absence of IGF-II; and 3) Tyr27 of IGF-II is important for the ternary complex formation.

Structural changes in insulin-like growth factor (IGF) I mutant proteins affecting binding kinetic rates to IGF binding protein 1 and IGF-I receptor

Ligand binding properties of five single amino acid substituted variants (V11A, D12A, Q15A, Q15E, and F16A) of human insulin-like growth factor I (IGF-I) were analyzed with respect to their binding affinities and binding kinetics to recombinant IGF binding protein 1 (IGFBP-1) and a soluble form of the IGF type I receptor (sIGF-I(R)), respectively. Side chains of the substituted residues are all predicted to be the most surface exposed in the alpha-helical portion of the B-region of the IGF-I molecule. The IGF-I variants were produced as fusion proteins to a IgG(Fc) binding protein domain, Z. Ligand binding kinetic rates were determined using BIAcore biosensor interaction analysis technology. All IGF-I variants showed altered binding affinities to both IGFBP- I and sIGF-I(R). Secondary structure content of the IGF-I variants was estimated using far-UV circular dichroism spectroscopy, followed by variable selection secondary structure calculations. The amount of calculated alpha-helicity is reduced for all the mutants, most predominantly for IGF-I(V11A) and IGF-I(F16A) proteins. Surprisingly, most of the effects of reduced binding affinities to both target proteins are attributed to lowered on-rates of binding, and these are correlated with the amount of alpha-helicity in each IGF-I variant. In addition, in some of the IGF-I variants, lowered off-rates of binding are observed. From the results, we propose that IGF-I is unusually sensitive to structural changes by surface amino acid substitutions in the B-region of the molecule. Therefore, biochemical or biological properties of amino acid substituted variants of IGF-I cannot be used in a straightforward way to dissect the direct involvement in binding of individual amino acid residues since structural changes may be involved.

Biophysical and biological properties of naturally occurring high molecular weight insulin-like growth factor II variants

A soluble form of the insulin-like growth factor II/mannose 6-phosphate receptor (sIGF-II/MPR) is present in fetal bovine serum and carries mature 7.5-kDa insulin-like growth factor II (IGF-II) and at least 12 different high molecular weight (Mr) IGF-II isoforms (Valenzano, K. J., Remmler, J., and Lobel, P. (1995) J. Biol. Chem. 270, 16441-16448). In this study, we used gel filtration and anion exchange chromatographies to resolve the isoforms into eight fractions that were characterized with respect to their biochemical, biophysical, and biological properties. Each fraction contained one to three major protein species with apparent sizes ranging from 11 to 17 kDa by SDS-polyacrylamide gel electrophoresis. The 11-kDa species contains no post-translational modifications and consists of an extended IGF-II backbone terminating at Gly-87. The remaining high Mr IGF-II isoforms are also composed of an 87-amino acid IGF-II peptide backbone but contain increasing amounts of sialated, O-linked sugars. Plasmon resonance spectroscopy experiments revealed that all the high Mr isoforms and mature 7.5-kDa IGF-II bound to immobilized recombinant soluble human IGF-I receptor, recombinant human IGF-binding protein 1, and sIGF-II/MPR with similar kinetics. In addition, radiolabeled tracer experiments demonstrated that both mature and high Mr IGF-II isoforms have similar binding profiles in fetal bovine serum and have similar affinities for IGF-II-binding proteins secreted from human fibroblasts. Finally, the biological activity of high Mr IGF-II was shown to be similar to or slightly better than mature IGF-II in stimulating amino acid uptake in fibroblasts and in inducing myoblast differentiation.

The insulin-like growth factor (IGF) binding site of bovine insulin-like growth factor binding protein-2 (bIGFBP-2) probed by iodination

The insulin-like growth factor (IGF) binding site of bovine insulin-like growth factor binding protein 2 (bIGFBP-2) has been probed by chemical iodination. Tyrosyl residues of bIGFBP-2 were reacted by chloramine T-mediated iodination. The modification patterns of free bIGFBP-2 and bIGFBP-2 associated with insulin-like growth factor II (IGF-II) were compared by tryptic mapping using electrospray mass spectrometry and N-terminal sequencing. The presence of bound IGF-II resulted in protection of tyrosine at position 60 from iodination measured by the relative loss of tyrosine specific fluorescence and the incorporation of the radioisotope 125I. In addition, the pattern of iodine incorporation of bIGFBP-2 was not different whether IGF-I or IGF-II was the protective ligand. bIGFBP-2, when iodinated alone sustained a 8-fold loss of binding affinity for IGF-I and a 4-fold loss in binding affinity for IGF-II. In contrast, bIGFBP-2 iodinated while complexed with either IGF-I or IGF-II retained the same binding affinity for IGF-I or IGF-II as non-iodinated bIGFBP-2. We conclude that tyrosine 60 lies either in a region of bIGFBP-2 which directly interacts with both IGF-I and IGF-II or lies in a region of bIGFBP-2 which undergoes a conformational change that is important for IGF binding. Furthermore, iodination of tyrosine residues at positions 71, 98, 213, 226, and 269 has no detectable impact on binding of bIGFBP-2 to the IGFs.

N-terminal deletion mutants of insulin-like growth factor-II (IGF-II) show Thr7 and Leu8 important for binding to insulin and IGF-I receptors and Leu8 critical for all IGF-II functions

To define the role of the N-terminal region of insulin-like growth factor-II (IGF-II) in its binding to insulin and IGF receptors, deletion mutants des-(1-5)-, des-(1-7)-, and des-(1-8)-recombinant (r) IGF-II, and the Gly8 for Leu substitution mutant of rIGF-II were prepared by site-directed mutagenesis, expressed in Escherichia coli, and purified. The binding affinity and mitogenic activity of these rIGF-II mutants as well as commercially available des-(1-6)-rIGF-II were analyzed. While the relative affinity of des-(1-5)- and des-(1-6)-rIGF-II for purified human insulin and IGF-I receptors remained at > or = 50% levels of that of rIGF-II, the affinity of des-(1-7)-rIGF-II decreased to approximately 10% and approximately 3%, respectively, of that of rIGF-II. When the octapeptide including Leu8 was removed prior to the Cys9-Cys47 intrachain bond, the relative affinity of this deletion mutant, des-(1-8)-rIGF-II, for these receptors dramatically decreased to < 1% of that of rIGF-II. Substituting Gly8 for Leu in rIGF-II decreased the affinity of this mutant for the IGF-I and insulin receptors to about the same extent. These results suggest that the side chains of Thr7 and Leu8 may play an important role in retaining all of the IGF-II functions. Decreases in the relative affinity for binding of the mutants to these receptors paralleled the decreases in their mitogenic potency for cultured Balb/c 3T3 cells. Although the relative affinity of des-(1-8)- or [Gly8]rIGF-II for rat IGF-II/CIM6-P (cation-independent mannose 6-phosphate) receptors was also < 1% of that of rIGF-II, the relative affinities of des-(1-5)-, des-(1-6)-, and des-(1-7)-rIGF-II for these receptors was significantly greater than that of rIGF-II. These results clearly demonstrate that Thr7 and Leu8 are important for binding to insulin and IGF-I receptors and Leu8 is critical for expression of all IGF-II functions.

Localization of the insulin-like growth factor II binding site to amino acids 1508-1566 in repeat 11 of the mannose 6-phosphate/insulin-like growth factor II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) binds insulin-like growth factor II (IGF-II) with high affinity. To localize the IGF-II binding site within the 15 repeating units that form the extracytoplasmic domain of the receptor, purified human M6P/IGF-II receptor was digested with thermolysin, and the fragments were analyzed for their ability to bind 125I-IGF-II in a cross-linking assay. Two IGF-II-binding receptor fragments of 23 and 37 kDa were purified. Sequence analysis revealed that the fragments consist of disulfide connected peptides comprising amino acids 1331-1566 and 1331-1697 of the receptor repeats 9-12. In a second approach we expressed truncated forms of the M6P/IGF-II receptor fused to the C terminus of the extracytoplasmic domain of the 46-kDa mannose 6-phosphate receptor. Fusion proteins containing M6P/IGF-II receptor repeats 10-15, 10-11, or 11-15 bound IGF-II, whereas a fusion protein containing the single repeat 10 failed to bind. This result indicates that repeat 11 (amino acids 1508-1650) is sufficient for binding of IGF-II. Residues 1508-1566, which are shared by the 23-kDa IGF-II-binding fragment and repeat 11, are proposed to form the IGF-II binding site of the M6P/IGF-II receptor.