Secretory protein translocation in a neurospora crassa in vitro system. Hydrolysis of a nucleoside triphosphate is required for posttranslational translocation

A cell-free translation-translocation system reconstituted with subcellular fractions from the wall-less variant fz;sg;os-1V of Neurospora crassa

A translation-translocation system reconstituted with subcellular fractions from the wall-less variant fz;sg;os-1V of Neurospora crassa reproduces in vitro translocation and processing of a secretory protein. The translation extract was isolated from the wall-less variant by gently lysing cells by a freeze-thaw procedure. This method yielded more extract then the method developed previously (R. Addison, J. Biol. Chem. 262: 17031, 1987) as well as reducing microsomal contamination. The microsomal fraction was isolated from lysed cells using a series of discontinuous sucrose gradients. The resultant microsomes were less inhibitory to translation of various transcripts and consisted of a more homogenous mixture of vesicles then microsomes prepared previously. Polyclonal antibodies directed against a polypeptide of approximately 75 kDa from the microsomes were used in indirect-immunofluorescence microscopy. The resultant fluorescent pattern shows a network of tubulo-reticular structures in a juxtanuclear region, which is the pattern expected of the rough endoplasmic reticulum.

Arginine-specific regulation mediated by the Neurospora crassa arg-2 upstream open reading frame in a homologous, cell-free in vitro translation system

Translational control mediated by an upstream open reading frame (uORF) in the 5'-leader of the Neurospora crassa arg-2 mRNA was reconstituted in a homologous, cell-free in vitro translation system. A cell-free N. crassa system was developed that required the presence of cap and poly(A) on RNA for maximal translation and that was amino acid-dependent. The 24-codon arg-2 uORF, when placed in the 5'-leader region of capped and adenylated synthetic luciferase RNAs, conferred Arg-specific negative regulation in this system. Improving the uORF translation initiation context decreased luciferase production and only slightly increased the magnitude of Arg-specific regulation. Mutation of uORF Asp codon 12 to Asn, which eliminates Arg-specific regulation in vivo, eliminated regulation in vitro. Elimination of the uORF translation initiation codon also eliminated Arg-specific regulation. Arg-specific regulation in vitro appeared to be reversible. Control of RNA stability did not appear to be a primary component of Arg-specific regulation in vitro. Comparison of the effects of adding Arg to in vitro translation reactions with adding compounds related to Arg indicated that Arg-specific translational regulation was specific for L-arginine.

A novel integration signal that is composed of two transmembrane segments is required to integrate the Neurospora plasma membrane H(+)-ATPase into microsomes

The Neurospora plasma membrane H(+)-ATPase belongs to a family of cation-motive porters called P-type ATPases. Putative transmembrane segments of these enzymes contain one or more charged residues. Conditions were determined by which a transmembrane segment with charged residues is integrated into its cognate membrane. We constructed fusion proteins flanked by the hydrophilic domains of the amino and carboxyl termini of the H(+)-ATPase that contained either one or two transmembrane segments. Neurospora in vitro translation system supplemented with homologous microsomes was programmed with RNA transcripts of these constructs. When transmembrane segment number one (M1) or number two (M2) of the H(+)-ATPase was engineered into the construct, the resultant protein did not integrate into microsomes. When M1 and M2 were placed in tandem, the resultant protein integrated into microsomes as judged by the criteria of resistance to extraction at pH 11.5 and protection from protease digestion. The integration event depended on ATP and GTP and on microsomal protein(s). We posited that membrane topology of the amino-terminal third of the H(+)-ATPase, and perhaps of other P-type ATPases is achieved by inserting transmembrane segments into membrane in pairs.

The membrane topology of the carboxyl-terminal third of the Neurospora plasma membrane H(+)-ATPase

To localize transmembrane segments in the carboxyl-terminal third of the Neurospora plasma membrane H(+)-ATPase, we constructed fusion proteins on the cDNA level. These contained DNA fragments encoding hydrophilic residues of the amino and carboxyl termini of the H(+)-ATPase with a DNA fragment encoding the putative transmembrane segment. To report translocation into microsomes, a DNA fragment encoding three consensus N-linked glycosylation sites was engineered carboxyl-terminal to the putative transmembrane segment. Fusion proteins were synthesized in a Neurospora in vitro translation system supplemented with homologous microsomes. By the criteria of glycosylation of fusion proteins by microsomes, sedimentation of products with microsomes after alkaline extraction, and analysis of protected fragments generated from proteinase K digestion of integrated products, we localized six transmembrane segments in the carboxyl-terminal third of the H(+)-ATPase. These results support a 10-segment model of the Neurospora H(+)-ATPase.

The initial association of a truncated form of the Neurospora plasma membrane H(+)-ATPase and of the precursor of yeast invertase with microsomes are distinct processes

Translocation and integration activities were assessed in Neurospora microsomes (nRM) after modification either by a sulfhydryl alkylating reagent or by a proteinase. A Neurospora in vitro system was programmed with RNA transcripts that encode the amino-terminal 194 amino-acid residues of the Neurospora plasma membrane H(+)-ATPase (pma194+) or the 262 amino-acid residues of the precursor of yeast invertase (preinv262). The processing of preinv262 was blocked in N-phenylmaleimide- and in trypsin-pretreated nRM. In contrast, the binding of preinv262 to microsomes was unaffected in the chemically alkylated nRM, but was affected in the trypsin-pretreated nRM. In the chemically alkylated vesicles, the integration of the pma194+ was not affected, but was partially blocked in the trypsin-pretreated vesicles. These data imply that trypsin-sensitive components are required for these activities in nRM, and that binding, translocation and integration can be differentiated by their sensitivity to chemical alkylation of sulfhydryl groups in nRM. Evaluated also were the effects of temperature on translocation and integration activities in the nRM. These were maximal at 20 degrees C, whereas the binding of preinv262 was maximal at 0 degree C. Taken together, these data demonstrate that the processing of preinv262 by nRM can be resolved into two steps: binding of the precursor protein to nRM and subsequent translocation into the lumen of the vesicles. Whereas, the integration of the pma194+ into nRM could not be resolved into separable steps. Taken together, these results are interpreted to imply that the initial association of truncated forms of the pma+ and the precursor of invertase to the surface of the nRM are distinct processes.

Enhancement of transforming potential of human insulinlike growth factor 1 receptor by N-terminal truncation and fusion to avian sarcoma virus UR2 gag sequence

The human insulinlike growth factor 1 (hIGF-1) receptor (hIGFR) is a transmembrane protein tyrosine kinase (PTK) molecule which shares high sequence homology in the PTK domain with the insulin receptor and, to a lesser degree, the ros transforming protein of avian sarcoma virus UR2. To assess the transforming potential of hIGFR, we introduced the intact and altered hIGFR into chicken embryo fibroblasts (CEF). The full-length hIGFR cDNA (fIGFR) was cloned into a UR2 retroviral vector, replacing the original oncogene v-ros. fIGFR was able to promote the growth of CEF in soft agar and cause morphological alteration in the absence of added hIGF-1 to medium containing 11% calf and 1% chicken serum. The transforming ability of hIGFR was not further increased in the presence of 10 nM exogenous hIGF-1. The 180-kDa protein precursor of hIGFR was synthesized and processed into alpha and beta subunits. The overexpressed hIGFR in CEF bound hIGF-1 with high affinity (Kd = 5.4 x 10(-9) M) and responded to ligand stimulation with increased tyrosine autophosphorylation. The cDNA sequence coding for part of the beta subunit of hIGFR, including 36 amino acids of the extracellular domain and the entire transmembrane and cytoplasmic domains, was fused to the 5' portion of the gag gene in the UR2 vector to form an avian retrovirus. The resulting virus, named UIGFR, was able to induce morphological transformation and promote colony formation of CEF with a stronger potency than did fIGFR. The UIGFR genome encodes a membrane-associated, glycosylated gag-IGFR fusion protein. The specific tyrosine phosphorylation of the mature form of the fusion protein, P75, is sixfold higher in vitro and threefold higher in vivo than that of the native IGFR beta subunit, P95. In conclusion, overexpression of the native or an altered hIGFR can induce transformation of CEF with the gag-IGFR fusion protein possessing enhanced transforming potential, which is consistent with its increased in vitro and in vivo tyrosine phosphorylation.

GTP is required for the integration of a fragment of the Neurospora crassa H(+)-ATPase into homologous microsomal vesicles

The integration of a fragment of the Neurospora crassa plasma membrane H(+)-ATPase was examined to determine if insertion of the fragment into homologous microsomal vesicles is obligatorily dependent on a nucleoside triphosphate. RNA transcripts that encoded the amino terminal 344 amino acids of the Neurospora crassa plasma membrane H(+)-ATPase(pma(344)+) were translated in a N. crassa in vitro system. The pma(344)+ integrated post-translationally into homologous microsomal vesicles independent of the associated ribosomes and dependent on the presence of GTP or guanylyl imidodiphosphate, a nonhydrolyzable analogue of GTP. ATP or analogues thereof did not support the integration of pma(344)+ into nRM post-translationally. These results were interpreted to suggest that a GTPase plays an essential role in the integration of the amino terminal portion of the pma+ into the endoplasmic reticulum.

Protein transport and compartmentation in yeast

Many newly synthesized proteins must be translocated across one or more membranes to reach their destination in the individual organelles or membrane systems. Translocation, mostly requiring an energy source, a signal on the protein itself, loose conformation of the protein and the presence of cytosolic and/or membrane receptor-like proteins, is often accompanied by covalent modifications of transported proteins. In this review I discuss these aspects of protein transport via the classical secretory pathway and/or special translocation mechanisms in the unicellular eukaryotic organism Saccharomyces cerevisiae.

Role of gag sequence in the biochemical properties and transforming activity of the avian sarcoma virus UR2-encoded gag-ros fusion protein

The transforming protein P68gag-ros of avian sarcoma virus UR2 is a transmembrane tyrosine protein kinase molecule with the gag portion protruding extracellularly. To investigate the role of the gag moiety in the biochemical properties and biological functions of the P68gag-ros fusion protein, retroviruses containing the ros coding sequence of UR2 were constructed and analyzed. The gag-free ros protein was expressed from one of the mutant retroviruses at a level 10 to 50% of that of the wild-type UR2. However, the gag-free ros-containing viruses were not able to either transform chicken embryo fibroblasts or induce tumors in chickens. The specific tyrosine protein kinase activity of gag-free ros protein is about 10- to 20-fold reduced as judged by in vitro autophosphorylation. The gag-free ros protein is still capable of associating with membrane fractions including the plasma membrane, indicating that sequences essential for recognition and binding membranes must be located within ros. Upon passages of the gag-free mutants, transforming and tumorigenic variants occasionally emerged. The variants were found to have regained the gag sequence fused to the 5' end of the ros, apparently via recombination with the helper virus or through intramolecular recombination between ros and upstream gag sequences in the same virus construct. All three variants analyzed code for gag-ros fusion protein larger than 68 kDa. The gag-ros recombination junction of one of the transforming variants was sequenced and found to consist of a p19-p10-p27-ros fusion sequence. We conclude that the gag sequence is essential for the transforming activity of P68gag-ros but is not important for its membrane association.

Studies on the sedimentation behavior of the Neurospora crassa plasma membrane H(+)-ATPase synthesized in vitro and integrated into homologous microsomal membranes

RNA transcripts that encoded the Neurospora crassa plasma membrane H(+)-ATPase (pma+), a polytopic integral membrane protein, and the pma+344, a truncated pma+ with the amino terminal 344 amino acids, were translated in a N. crassa in vitro system. The microsomal membranes integrated products were insensitive to extraction by Na2CO3 (pH 11.5). The velocity sedimentation behavior of the in vitro synthesized pma+ were examined under various conditions. The pma+ migrated on linear sucrose gradients as aggregates which were heterogeneous in size, in the regions of 9-13 S; whereas, these values were reduced when Triton X-100 was presence in the gradients. The formation of these aggregates is interpreted to suggest a mechanism that maintains this polytopic integral membrane protein in a soluble form until it is targeted to the membranes. The sedimentation coefficient of the Triton X-100 solubilized microsomal membranes integrated pma+ corresponded roughly to a monomer of the pma+. Furthermore, a comparison of the trypsin cleavage patterns of the in vitro synthesized pma+ and of the microsomal membranes integrated pma+ suggest that they have different tertiary, or quaternary, structures. The latter did not give the characteristic trypsin cleavage patterns that have been observed for the native pma+ in the presence of its ligands MgATP and vanadate (Addison, R. and Scarborough, G.A. (1982) J. Biol. Chem. 257, 10421-10426). This was interpreted to suggest that the microsomal membranes integrated pma+ cannot interact with its substrate, suggesting that it is catalytically inactive.

In vitro studies on the translocation of acid phosphatase into the endoplasmic reticulum of the yeast Saccharomyces cerevisiae

We demonstrate here the in vitro translocation of yeast acid phosphatase into rough endoplasmic reticulum. The precursor of the repressible acid phosphatase from Saccharomyces cerevisiae encoded by the PHO5 gene, was synthesized in a yeast lysate programmed with in vitro transcribed PHO5 mRNA. In the presence of yeast rough microsomes up to 16% of the acid phosphatase synthesized was found to be translocated into the microsomes, as judged by proteinase resistance, and fully core-glycosylated. The translocation efficiency however, decreased to 3% if yeast rough microsomes were added after synthesis of acid phosphatase had been terminated. When a wheat-germ extract was used for in vitro synthesis, the precursor of acid phosphatase was translocated into canine pancreatic rough microsomes and thereby core-glycosylated in a signal-recognition-particle-dependent manner. Replacing canine with yeast rough microsomes in the wheat-germ translation system, however, resulted in a significant decrease in the ability to translocate and glycosylate the precursor. Translocation and glycosylation were partially restored by a high-salt extract prepared from yeast ribosomes. The results presented here suggest that yeast-specific factors are needed to translocate and glycosylate acid phosphatase efficiently in vitro.

70K heat shock related proteins stimulate protein translocation into microsomes

A yeast cytosol is shown to contain two distinct activities that stimulate protein translocation across microsomal membranes. One activity was purified. It consists of two constitutively expressed 70K heat shock related proteins that increase the rate of translocation. Possible mechanisms of action of these proteins are discussed.