Molecular cloning of human fibroblast hyaluronic acid-binding protein confirms its identity with P-32, a protein co-purified with splicing factor SF2. Hyaluronic acid-binding protein as P-32 protein, co-purified with splicing factor SF2

Crystal structure of human p32, a doughnut-shaped acidic mitochondrial matrix protein

Human p32 (also known as SF2-associated p32, p32/TAP, and gC1qR) is a conserved eukaryotic protein that localizes predominantly in the mitochondrial matrix. It is thought to be involved in mitochondrial oxidative phosphorylation and in nucleus-mitochondrion interactions. We report the crystal structure of p32 determined at 2.25 A resolution. The structure reveals that p32 adopts a novel fold with seven consecutive antiparallel beta-strands flanked by one N-terminal and two C-terminal alpha-helices. Three monomers form a doughnut-shaped quaternary structure with an unusually asymmetric charge distribution on the surface. The implications of the structure on previously proposed functions of p32 are discussed and new specific functional properties are suggested.

Molecular studies into the role of CD44 variants in metastasis in gastric cancer

CD44, an integral membrane glycoprotein expressed by many cell types, serves as the principal transmembrane hyaluronate receptor and might be a determinant of metastatic and invasive behaviour in carcinomas. The generation of CD44 splice variants might be linked closely with gastric carcinoma tumorigenesis and differentiation. Some studies have reported that the magnitude of CD44 variant synthesis at the protein level correlates with lymph node metastasis. A number of studies have examined the possible mechanism of involvement of the CD44 variant in tumour metastasis. Most studies have reported that the regulation of CD44 binding to hyaluronate results from glycosylation of variably spliced exons. Direct hyaluronate binding studies of CD44 V4-V7 isoforms transfected into the human gastric carcinoma cell line, SC-M1, have indicated that the V4-V7 isoforms themselves, in addition to glycosylation, can alter hyaluronate binding.

Rapid hyaluronan uptake is associated with enhanced motility: implications for an intracellular mode of action

Texas red-labeled hyaluronan (TR-HA) is rapidly taken up in a CD44 independent manner into ras-transformed 10T1/2 fibroblasts, where it accumulates in both cell ruffles/lamellae, the perinuclear area, and the nucleus. HA does not accumulate in the cell ruffles/lamellae of parental 10T1/2 cells. Addition of HA to ras-transformed cells promotes their random motility but has no effect on 10T1/2 cell motility. 10T1/2 cells can be modified to take up HA into cell ruffles by exposure to phorbol ester or direct microinjection of HA into cells. Both treatments significantly stimulate 10T1/2 cell motility.

Identification of IHABP, a 95 kDa intracellular hyaluronate binding protein

The extracellular matrix component hyaluronan is believed to play important roles in various processes of organogenesis, cell migration and cancer. Recognition of and binding to hyaluronan is mediated by cell surface receptors. Three of them, CD44, ICAM-1 and RHAMM (receptor for hyaluronic acid mediated motility), have been identified. A cDNA clone designated RHAMM turned out to possess transforming capacity. Based on this published sequence, we isolated the complete cDNA of the murine gene. The cDNA comprises an open reading frame of 2.3 kb and encodes a 95 kDa protein. The protein carries a hyaluronan binding motif which binds to hyaluronan in vitro but not to heparin or chondroitin sulphate. It is ubiquitously expressed in normal cells and in all tumour cell lines irrespective of their metastatic properties. One tumour cell line, the metastatic Lewis lung carcinoma, expresses a larger 105 kDa variant form of the protein due to a genomic rearrangement. Antibodies raised against the 95 kDa protein were used for subcellular localization studies. The hyaluronan binding protein is not detectable at the cell surface but is rather localized exclusively intracellularly. Clearly, the sequence we have identified encodes a protein with properties substantially different to the RHAMM protein. We tentatively name the protein intracellular hyaluronic acid binding protein, IHABP.

Glypican and biglycan in the nuclei of neurons and glioma cells: presence of functional nuclear localization signals and dynamic changes in glypican during the cell cycle

We have investigated the expression patterns and subcellular localization in nervous tissue of glypican, a major glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan that is predominantly synthesized by neurons, and of biglycan, a small, leucine-rich chondroitin sulfate proteoglycan. By laser scanning confocal microscopy of rat central nervous tissue and C6 glioma cells, we found that a significant portion of the glypican and biglycan immunoreactivity colocalized with nuclear staining by propidium iodide and was also seen in isolated nuclei. In certain regions, staining was selective, insofar as glypican and biglycan immunoreactivity in the nucleus was seen predominantly in a subpopulation of large spinal cord neurons. The amino acid sequences of both proteoglycans contain potential nuclear localization signals, and these were demonstrated to be functional based on their ability to target beta-galactosidase fusion proteins to the nuclei of transfected 293 cells. Nuclear localization of glypican beta-galactosidase or Fc fusion proteins in transfected 293 cells and C6 glioma cells was greatly reduced or abolished after mutation of the basic amino acids or deletion of the sequence containing the nuclear localization signal, and no nuclear staining was seen in the case of heparan sulfate and chondroitin sulfate proteoglycans that do not possess a nuclear localization signal, such as syndecan-3 or decorin (which is closely related in structure to biglycan). Transfection of COS-1 cells with an epitope-tagged glypican cDNA demonstrated transport of the full-length proteoglycan to the nucleus, and there are also dynamic changes in the pattern of glypican immunoreactivity in the nucleus of C6 cells both during cell division and correlated with different phases of the cell cycle. Our data therefore suggest that in certain cells and central nervous system regions, glypican and biglycan may be involved in the regulation of cell division and survival by directly participating in nuclear processes.

Regulation of cellular phosphorylation of hyaluronan binding protein and its role in the formation of second messenger

The presence of the 34-kDa hyaluronan binding protein, a new member of the 'hyaladherins' family, was demonstrated in a wide variety of cell lines by immunoblot analysis. This protein was observed to be highly phosphorylated in transformed fibroblasts compared to normal fibroblasts. Phosphorylation was enhanced in the presence of its ligand i.e., hyaluronan, but not in the presence of other glycosaminoglycans. The phosphorylated form of this hyaluronan binding protein was shown to be present on the cell surface and could be detected in serum-free medium. The regulation of the cellular and cell surface phosphorylation of HA-binding protein by HA, PMA and calyculin-A was demonstrated in different cell lines. Hyaluronan enhanced the phosphorylation of PLC-gamma in association with increased formation of inositol 1,4,5-triphosphate, both of which were specifically blocked by pretreatment of the cells with purified anti-hyaluronan binding protein antibodies. The data presented here indicate a role for the 34-kDa hyaluronan binding protein in cellular signal transduction.

p32 protein, a splicing factor 2-associated protein, is localized in mitochondrial matrix and is functionally important in maintaining oxidative phosphorylation

Human p32, originally cloned as a splicing factor 2-associated protein, has been reported to interact with a variety of molecules including human immunodeficiency virus Tat and complement 1q (C1q). p32 protein is supposed to be in the nucleus and on the plasma membrane for the association with human immunodeficiency virus Tat and C1q, respectively. None of the interactions, however, is proven to have a physiological role. To investigate the physiological function of p32, we determined the intracellular localization of p32. The fractionation of cells, fluorescent immunocytochemistry, and electron microscopic immunostaining show that p32 is exclusively localized in the mitochondrial matrix. We cloned a Saccharomyces cerevisiae homologue of human p32 gene, referred to yeast p30 gene. The yeast p30 protein is also localized in the mitochondrial matrix. The disruption of the p30 gene caused the growth retardation of yeast cells in a glycerol medium but not in a glucose medium, i.e. the impairment of the mitochondrial ATP synthesis. The growth impairment was restored by the introduction of the human p32 cDNA, indicating that p30 is a functional yeast counterpart of human p32. Taken together, both p32 and p30 reside in mitochondrial matrix and play an important role in maintaining mitochondrial oxidative phosphorylation.

P32/TAP, a cellular protein that interacts with EBNA-1 of Epstein-Barr virus

The Epstein-Barr virus (EBV) EBNA-1 protein has a central role in the maintenance of a latent EBV infection and is the only virus-encoded protein expressed in all EBV-associated tumors. EBNA-1 is required for replication of the episomal form of the latent viral genome and transactivates the latency C and LMP-1 promoters. The mechanisms by which EBNA-1 performs these functions are not known. Here we describe the cloning, expression, and characterization of a cellular protein, P32/TAP, which strongly interacts with EBNA-1. We show that P32/TAP is expressed at high levels in Raji cells and is synthesized as a proprotein of 282 amino acids (aa) that is posttranslationally processed by a two-step cleavage process to yield a mature protein of 209 aa. It has been previously reported that P32/TAP is expressed on the cell surface. Our transient expression assays detected full-length P32/TAP (1-282 aa) in the cytoplasm while mature P32/TAP protein localized to the nucleus. Three lines of evidence support P32/TAP interaction with EBNA-1. First, in the yeast two-hybrid system we mapped two interactive N-terminal regions of EBNA-1, aa 40-60 and aa 325-376, each of which contains arginine-glycine repeats. These regions interact with the C-terminal half of P32/TAP. Second, the full-length cytoplasmic P32/TAP protein can translocate nuclear EBNA-1 into the cytoplasm. Third, P32/TAP co-immunoprecipitated with EBNA-1. We have confirmed that a Gal4 fusion protein containing the C-terminal region of P32/TAP (aa 244-282) transactivates expression from a reporter containing upstream Gal4-binding sites. Deletion of the P32/TAP interactive regions of EBNA-1 severely diminished EBNA-1 transactivation of FRTKCAT in transient expression assays. Our data suggest that interaction with P32/TAP may contribute to EBNA-1-mediated transactivation.

Multifunctional activities of human fibroblast 34-kDa hyaluronic acid-binding protein

We have already reported that human fibroblast 34-kDa hyaluronic acid-binding protein (HABP) is identical with P32, the protein co-purified with splicing factor SF-2 [Deb and Datta (1996) J. Biol. Chem. 271, 2206-2212]. Data search further revealed that it has 92% sequence homology with a murine protein YL2 which interacts with HIV1 Rev. In this paper we have successfully demonstrated that HIV1 Rev binds with labeled 34-kDa HABP which can be competed with excess unlabeled HABP, suggesting this protein can be a cellular factor promoting HIV1 Rev to function. Interestingly, the multifunctional nature of HABP has been elucidated as it has 100% homology with another protein gC1q, the complement protein. The distinct non-overlapping binding motifs for HA and gC1q have been identified in the same protein, suggesting that either the protein can function independently or its activity is regulated by ligand binding, wherein its binding to one of the ligands may modulate the receptor activity of the other ligand.

Interpreting cDNA sequences: some insights from studies on translation

This review discusses some rules for assessing the completeness of a cDNA sequence and identifying the start site for translation. Features commonly invoked-such as an ATG codon in a favorable context for initiation, or the presence of an upstream in-frame terminator codon, or the prediction of a signal peptide-like sequence at the amino terminus-have some validity; but examples drawn from the literature illustrate limitations to each of these criteria. The best advice is to inspect a cDNA sequence not only for these positive features but also for the absence of certain negative indicators. Three specific warning signs are discussed and documented: (i) The presence of numerous ATG codons upstream from the presumptive start site for translation often indicates an aberration (sometimes a retained intron) at the 5' end of the cDNA. (ii) Even one strong, upstream, out-of-frame ATG codon poses a problem if the reading frame set by the upstream ATG overlaps the presumptive start of the major open reading frame. Many cDNAs that display this arrangement turn out to be incomplete; that is, the out-of-frame ATG codon is within, rather than upstream from, the protein coding domain. (iii) A very weak context at the putative start site for translation often means that the cDNA lacks the authentic initiator codon. In addition to presenting some criteria that may aid in recognizing incomplete cDNA sequences, the review includes some advice for using in vitro translation systems for the expression of cDNAs. Some unresolved questions about translational regulation are discussed by way of illustrating the importance of verifying mRNA structures before making deductions about translation.