Deletion of the pro-alpha 1(I) N-propeptide affects secretion of type I collagen in Chinese hamster lung cells but not in Mov-13 mouse cells

MMP-12 catalytic domain recognizes and cleaves at multiple sites in human skin collagen type I and type III

Collagens of either soft connective or mineralized tissues are subject to continuous remodeling and turnover. Undesired cleavage can be the result of an imbalance between proteases and their inhibitors. Owing to their superhelical structure, collagens are resistant to many proteases and matrix metalloproteinases (MMPs) are required to initiate further degradation by other enzymes. Several MMPs are known to degrade collagens, but the action of MMP-12 has not yet been studied in detail. In this work, the potential of MMP-12 in recognizing sites in human skin collagen types I and III has been investigated. The catalytic domain of MMP-12 binds to the triple helix and cleaves the typical sites -Gly(775)-Leu(776)- in alpha-2 type I collagen and -Gly(775)-Ile(776)- in alpha-1 type I and type III collagens and at multiple other sites in both collagen types. Moreover, it was observed that the region around these typical sites contains comparatively less prolines, of which some have been proven to be only partially hydroxylated. This is of relevance since partial hydroxylation in the vicinity of a potential scissile bond may have a local effect on the conformational thermodynamics with probable consequences on the collagenolysis process. Taken together, the results of the present work confirm that the catalytic domain of MMP-12 alone binds and degrades collagens I and III.

An Intramolecular Interaction between SH2-Kinase Linker and Kinase Domain Is Essential for the Catalytic Activity of Protein-tyrosine Kinase-6

Protein-tyrosine kinase-6 (PTK6, also known as Brk) is a non-receptor tyrosine kinase that contains SH3, SH2, and catalytic (Kinase) domains. We have identified an intramolecular interaction between the linker (Linker) region connecting the SH2 and Kinase domains and the Kinase domain. Residue Trp-184 within the Linker region is essential for the Linker-Kinase interaction but not for the Linker-SH3 interaction. A recombinant PTK6 Kinase domain connected to the Linker region had catalytic activity in terms of autophosphorylation, phosphorylation of a PTK6 substrate, BKS, and phosphorylation of an oligopeptide substrate, whereas the Kinase domain itself, or one connected to a Linker region containing a W184A substitution, did not. The introduction of the W184A mutation into PTK6 also abrogated autophosphorylation and phosphorylation of another PTK6 substrate, Sam68, as well as phosphorylation of intracellular proteins. It also abolished the ability of PTK6 to promote proliferation and prevent apoptosis of HEK 293 cells, as well as to permit anchorage-independent colony formation. Therefore, unlike Src family members, in which the Linker-Kinase interaction inhibits catalytic activity, in PTK6 this interaction has an essential positive role.

The NH(2)-terminal propeptides of fibrillar collagens: highly conserved domains with poorly understood functions

The impetus for this review comes from the recent finding that the absence of the majority of the non-triple-helical sequence in the NH(2)-terminal propeptide (N-propeptide) of the pro alpha 1(I) collagen chain fails to generate a significant phenotype in the mouse (Bornstein et al., J. Biol. Chem., 277:2605-2613, 2002). This result is in apparent conflict with those of numerous studies in vitro that have implicated the N-propeptide in a number of processes that are involved in the biogenesis, maturation and function of type 1 collagen. To seek an explanation for this discrepancy, the sequences of the highly conserved, 55-57-amino acid, cysteine-rich repeats (CRR), which constitute the majority of the globular domains in the N-propeptides, were compared among 13 vertebrate species. Surprisingly, the CRR in mice and rats differs substantially from those in other mammalian species. Indeed, the CRR in birds, fish and amphibia are more similar to those of other mammals than are the CRR in rodents. This finding raises the possibility that the mutant mouse, which lacks exon 2 that encodes the CRR in the N-propeptide, might not be an appropriate model in which to study the function of the N-propeptide in other mammals. Alternatively, compensation, possibly by procollagens II or III, could account for the mild phenotype of the exon 2-deleted mouse. Yet another possibility is that the CRR plays a developmental role in the mouse, akin to that recently proposed for the N-propeptide in type IIA procollagen, rather than a function in collagen biogenesis. Some support for the latter possibility is provided by the observation that, on one background, the breeding of heterozygous exon 2-deleted mice generated homozygous mutants at less than the expected frequency. Experiments to examine these possibilities are proposed.

The globular domain of the proalpha 1(I) N-propeptide is not required for secretion, processing by procollagen N-proteinase, or fibrillogenesis of type I collagen in mice

The globular domain in the NH(2)-terminal propeptide (N-propeptide) of the proalpha1(I) chain is largely encoded by exon 2 of the Col1a1 gene and has been implicated in a number of processes that are involved in the biogenesis, maturation, and function of type I collagen. These include intracellular chain association, transcellular transport and secretion, proteolytic processing of the precursor, feedback regulation of synthesis, and control of fibrillogenesis. However, none of these proposed functions has been firmly established. To evaluate the function of this procollagen domain we have used a targeted mutagenesis approach to generate mice that lack exon 2 in the Col1a1 gene. Mouse lines were established on both a mixed 129 OlaHsd/Sv and C57BL/6 background and a pure 129 OlaHsd/Sv background. Adult mice on the mixed background are normal in appearance and are fertile. To the extent that they have been studied, procollagen synthesis, secretion, and proteolytic processing are normal in these mice, and collagen fibrillogenesis is only slightly altered. However, breeding of heterozygous mutant mice on the 129 background generated homozygous mutants at only 64% of the expected frequency. These findings suggest that although the N-propeptide is not essential for collagen biogenesis in mice it may play some essential role during embryonic development.

Production of human type I collagen in yeast reveals unexpected new insights into the molecular assembly of collagen trimers

Substantial evidence supports the role of the procollagen C-propeptide in the initial association of procollagen polypeptides and for triple helix formation. To evaluate the role of the propeptide domains on triple helix formation, human recombinant type I procollagen, pN-collagen (procollagen without the C-propeptides), pC-collagen (procollagen without the N-propeptides), and collagen (minus both propeptide domains) heterotrimers were expressed in Saccharomyces cerevisiae. Deletion of the N- or C-propeptide, or both propeptide domains, from both proalpha-chains resulted in correctly aligned triple helical type I collagen. Protease digestion assays demonstrated folding of the triple helix in the absence of the N- and C-propeptides from both proalpha-chains. This result suggests that sequences required for folding of the triple helix are located in the helical/telopeptide domains of the collagen molecule. Using a strain that does not contain prolyl hydroxylase, the same folding mechanism was shown to be operative in the absence of prolyl hydroxylase. Normal collagen fibrils were generated showing the characteristic banding pattern using this recombinant collagen. This system offers new opportunities for the study of collagen expression and maturation.

Alpha2beta1 integrin recognition of the carboxyl-terminal propeptide of type I procollagen: integrin recognition and feed-back regulation of matrix biosynthesis are mediated by distinct sequences

It has recently been established that the carboxyl-terminal propeptide of type I collagen exert a feedback regulatory effect on extracellular matrix biosynthesis and that the propeptide bind to the alpha2beta1 integrin. This raises the intriguing hypothesis that the regulatory propeptide sequences exert their effects as a consequence of binding to the integrin. We show that recombinant alpha1(I) collagen chain C-terminal propeptide contains a binding site for the intact alpha2beta1 integrin and for a recombinant alpha2 integrin I domain, but not for the alpha1beta1 integrin, a structurally and functionally related collagen/laminin receptor. Additional studies employing a series of recombinant N-terminal and C-terminal deletion mutants, internal fragments of the propeptide, synthetic peptides, recombinant alpha2 integrin I domain and inhibitory monoclonal antibodies established that the previously identified sequences within the alpha1(I) C-terminal propeptide that mediate regulation of matrix biosynthesis are neither necessary nor sufficient for alpha2beta1 integrin binding. In contrast, the integrin recognition site is composed of a conformationally complex determinant located within a structurally distinct 115 amino acid region of the propeptide.

In vitro expression analysis of collagen biosynthesis and assembly

While the generalised pathway of collagen biosynthesis is well understood, the specific molecular interactions that drive chain recognition and assembly and the formation of tissue-specific extracellular supramolecular structures have not been elucidated. This review focuses on the use of in vitro collagen expression systems to explore some of these fundamental questions on the molecular basis of normal and mutant collagen assembly. Three in vitro expression/assembly systems are discussed. Firstly, a simple cell-free transcription/translation system to study the initial stages of collagen chain assembly. Secondly, a novel T7-driven high level expression system, using a recombinant vaccinia virus expressing T7 RNA polymerase, in transiently transfected cells which allows appropriate postranslational modification and collagen folding. Thirdly, the more complex questions of normal and mutant collagen extracellular matrix assembly are addressed by stable transfection and expression in cells which allow the formation of a 'tissue equivalent' matrix during long-term culture.

Relationships between translation of pro alpha1(I) and pro alpha2(I) mRNAs during synthesis of the type I procollagen heterotrimer

Final assembly of the procollagen I heterotrimeric molecule is initiated by interactions between the carboxyl propeptide domains of completed, or nearly completed nascent pro alpha chains. These interactions register the chains for triple helix folding. Prior to these events, however, the appropriate nascent chains must be brought within the same compartments of the endoplasmic reticulum (ER). We hypothesize that the co-localization of the synthesis of the nascent pro alpha1(I) and pro alpha2(I) chains results from an interaction between their translational complexes during chain synthesis. This has been investigated by studying the polyribosomal loading of the pro alpha-chain messages during in vitro translation in the presence and absence of microsomal membranes, and in cells which have the ability to synthesize the pro alpha1 homotrimer or the normal heterotrimer. Recombinant human pro alpha1(I) and pro alpha2(I) cDNAs were inserted into plasmids and then transcribed in vitro. The resulting RNAs were translated separately and in mixture in a cell-free rabbit reticulocyte lysate +/- canine pancreatic microsomes. Cycloheximide (100 mu g/ml) was added and the polysomes were collected and fractionated on a 15-50% sucrose gradient. The RNA was extracted from each fraction and the level of each chain message was determined by RT-PCR. Polysomes from K16 (heterotrimer-producing), W8 (pro alpha1(I) homotrimer), and A2' (heterotrimer + homotrimer) cells were similarly analyzed. Translations of the pro alpha1(I) and pro alpha2(I) messages proceeded independently in the cell-free, membrane-free systems, but were coordinately altered in the presence of membrane. The cell-free + membrane translation systems mimicked the behavior of the comparable cell polysome mRNA loading distributions. These data all suggest that there is an interaction between the pro alpha chain translational complexes at the ER membrane surface which temporally and spatially localize the nascent chains for efficient heteromeric selection and folding.

Secretion and matrix assembly of recombinant type VI collagen

A monomer of type VI collagen is composed of three different chains of 140 (alpha 1), 130 (alpha 2), and 250-350 kDa (alpha 3). Monomers assemble into dimers (6 chains) and tetramers (12 chains) that are stabilized by disulfide bonds and, once associated one to another, give rise to a microfilamentous network in close apposition with cell surfaces and banded collagen fibers. We have derived murine NIH/3T3 cell lines that were transfected with the cDNAs for the three chains and that constitutively expressed chicken type VI collagen. Cotransfection was efficient because, in three out of six isolated cell lines, all chicken chains were expressed. Southern blotting demonstrated that several copies of each cDNA were integrated approximately in equal number. Expression of the three polypeptide chains was consistent with the levels of the respective mRNAs. The three chicken chains assembled by disulfide bonding to form correctly folded triple helical aggregated composites with sizes corresponding to type VI collagen monomers, dimers, and tetramers. These functional recombinant assemblies were secreted and became incorporated into the extracellular matrix, where they formed an extensive fibrillar network.