The t complex polypeptide 1 (TCP-1) is associated with the cytoplasmic aspect of Golgi membranes

Identification of chaperonin particles in mammalian brain cytosol and of T-complex polypeptide 1 as one of their components

An approximately 950-kDa heteromeric particle was purified from guinea-pig and rat brain by sucrose gradient fractionation of post-mitochondrial supernatants. Further purification, by affinity chromatography on ATP-Sepharose and anion exchange FPLC on MonoQ, yielded a particle with typical chaperonin ultrastructure. One of the component polypeptides was recognized by a monoclonal antibody to murine T-complex polypeptide 1. Brain cytosolic chaperonin particles formed a binary complex with unfolded tubulin subunits. The polypeptide compositions of the cytosolic chaperonin particles appeared very similar between brain and testicular tissues of the same animal, but differed subtly between the guinea-pig and rat.

Protein folding in the cell: functions of two families of molecular chaperone, hsp 60 and TF55-TCP1

Two families of molecular chaperone, the hsp 60-GroEL family and the TF55-TCP1 family, have been discovered in evolutionarily related cellular compartments. A member of one of these families, hsp 60, has been shown to play a global role in polypeptide chain folding in mitochondria. We review here studies of both hsp 60 and other family members, discussing their essential physiological roles and mechanism of action.

Changes in architecture of the Golgi complex and other subcellular organelles during myogenesis

Myogenesis involves changes in both gene expression and cellular architecture. Little is known of the organization, in muscle in vivo, of the subcellular organelles involved in protein synthesis despite the potential importance of targeted protein synthesis for formation and maintenance of functional domains such as the neuromuscular junction. A panel of antibodies to markers of the ER, the Golgi complex, and the centrosome were used to localize these organelles by immunofluorescence in myoblasts and myotubes of the mouse muscle cell line C2 in vitro, and in intact single muscle fibers from the rat flexor digitorum brevis. Antibodies to the ER stained structures throughout the cytoplasm of both C2 myoblasts and myotubes. In contrast, the spatial relationship between nucleus, centrosome, and Golgi complex was dramatically altered. These changes could also be observed in a low-calcium medium that allowed differentiation while preventing myoblast fusion. Muscle fibers in vivo resembled myotubes except that the ER occupied a smaller volume of cytoplasm and no staining was found for one of the Golgi complex markers, the enzyme alpha-mannosidase II. Electron microscopy, however, clearly showed the presence of stacks of Golgi cisternae in both junctional and extrajunctional regions of muscle fibers. The perinuclear distribution of the Golgi complex was also observed in live muscle fibers stained with a fluorescent lipid. Thus, the distribution of subcellular organelles of the secretory pathway was found to be similar in myotubes and muscle fibers, and all organelles were found in both junctional and extrajunctional areas of muscle.

Structure and expression of the gene encoding mouse t-complex polypeptide (Tcp-1)

The nucleotide (nt) sequence of the structural gene (Tcp-1) encoding mouse t-complex polypeptide 1 (TCP-1) has been determined. The nt sequence extending to 10,043 bp shows that the Tcp-1 gene is divided into 12 exons, 11 introns and 5'- and 3'-flanking regions. The Tcp-1 gene has a tight cluster of major transcription start points (tsp). Two GC boxes, one CCAAT box and some other possible regulatory elements are located in the region upstream from the tsp, but no TATA box was found. Extending from the 5'-flanking region to the first intron, a CpG dinucleotide-rich cluster is located. In addition, Tcp-1 gene transcripts in mouse organs, embryos and cultured cells were analyzed by Northern blotting. The Tcp-1 mRNA is enriched not only in testes, but also in early post-implantation embryos and some cultured cell lines, as compared with mouse organs other than the testis. The amount of Tcp-1 mRNA in embryos decreases during development. These results suggest that the expression of the Tcp-1 gene may be regulated spatially and temporally in embryonic and adult mice by transcriptional control or by mRNA stability.

A cytoplasmic chaperonin that catalyzes beta-actin folding

We have isolated a cytoplasmic chaperonin based on its ability to catalyze the folding of denatured beta-actin. The cytoplasmic chaperonin is organized as a multisubunit toroid and requires Mg2+ and ATP for activity. The folding reaction proceeds via the rapid ATP-independent formation of a binary complex, followed by a slower ATP-dependent release of the native product. Electron microscopic observations reveal a striking structural change that occurs upon addition of Mg2+ and ATP. The eukaryotic cytoplasm thus contains a chaperonin that is functionally analagous to its prokaryotic, mitochondrial, and chloroplastic counterparts.

T-complex polypeptide-1 is a subunit of a heteromeric particle in the eukaryotic cytosol

The murine t-complex encodes t-complex polypeptide-1 (TCP1), which is constitutively expressed in almost all cells, and upregulated during spermatogenesis. Mammalian sequences have greater than 96% identity with each other, and greater than 60% identity with Drosophila melanogaster and yeast orthologues. TCP1 is essential in yeast, and is postulated to be the cytosolic mammalian equivalent of groEL. We report here that, in the native state, murine and human TCP1 is distributed throughout the cytosol as an 800K-950K hetero-oligomeric particle in association with four to six unidentified proteins and two Hsp70 heat-shock proteins. Negative-stain electron microscopy indicates that the structure is two stacked rings, 12-16 nm in diameter. Therefore, despite similarities with the chaperonin 60 proteins, these data indicate that TCP1 is biochemically and structurally unique. We suggest that TCP1 may represent one of a family of molecules in the eukaryotic cytosol involved in protein folding and regulated in part by their heteromeric associations.

TCP1 complex is a molecular chaperone in tubulin biogenesis

A role in folding of newly translated proteins in the cytosol of eukaryotes has been proposed for t-complex polypeptide-1 (TCP1), although its molecular targets have not yet been identified. Tubulin is a major cytosolic protein whose assembly into microtubules is critical to many cellular processes. Although numerous studies have focused on the expression of tubulin, little is known about the processes whereby newly translated tubulin subunits acquire conformations that enable them to form alpha-beta-heterodimers. We examined the biogenesis of alpha- and beta-tubulin in rabbit reticulocyte lysate, and report here that newly translated tubulin subunits entered a 900K complex in a protease-sensitive conformation. Addition of Mg-ATP, but not nonhydrolysable analogues, released the tubulin subunits as assembly-competent protein with a conformation that was relatively protease-resistant. The 900K complex purified from reticulocyte lysate contained as its major constituent a 58K protein that cross-reacted with a monoclonal antiserum against mouse TCP1. We conclude that TCP1 functions as a cytosolic chaperone in the biogenesis of tubulin.

Nucleotide sequence of a mouse Tcp-1 pseudogene: a nucleotide record for a t complex gene carried by an ancestor of the mouse

We have isolated clones of a processed pseudogene of mouse t complex polypeptide 1 (Tcp-1) and determined the nucleotide sequence of the pseudogene. The pseudogene was 1363 bp long and had no intron. The Tcp-1 pseudogene had 88.4% or 88.3% nucleotide identity to the mouse Tcp-1 cDNA of wild-type (Tcp-1b) or t haplotype (Tcp-1a), and 87.5% identity to the rat Tcp-1 cDNA. On 12 nucleotide positions where the open reading frames (ORFs) of mouse Tcp-1b and Tcp-1a cDNAs have bp substitutions, the Tcp-1 pseudogene had 6 bp identical to Tcp-1b, 5 bp identical to Tcp-1a and 1 bp not identical to neither. On ten amino acid positions where TCP-1B and TCP-1A polypeptides have substitutions, deduced amino acids of the Tcp-1 pseudogene had four amino acids identical to TCP-1B, five amino acids identical to TCP-1A and one amino acid identical to neither. These results suggest that the ancestral mouse Tcp-1 gene would have had no significant difference between the resemblance to Tcp-1b and that to Tcp-1a before they were diverged and that amino acids of TCP-1B and TCP-1A would have been substituted in similar high rates.

A molecular chaperone from a thermophilic archaebacterium is related to the eukaryotic protein t-complex polypeptide-1

There is evidence to suggest that components of archaebacteria are evolutionarily related to cognates in the eukaryotic cytosol. We postulated that the major heat-shock protein of the thermophilic archaebacterium, Sulfolobus shibatae, is a molecular chaperone and that it is related to an as-yet unidentified chaperone component in the eukaryotic cytosol. Acquired thermotolerance in S. shibatae correlates with the predominant synthesis of this already abundant protein, referred to as thermophilic factor 55 (TF55). TF55 is a homo-oligomeric complex of two stacked 9-membered rings, closely resembling the 7-membered-ring complexes of the chaperonins, groEL, hsp60 and Rubisco-binding protein. The TF55 complex binds unfolded polypeptides in vitro and has ATPase activity-features consistent with its being a molecular chaperone. The primary structure of TF55, however, is not significantly related to the chaperonins. On the other hand, it is highly homologous (36-40% identity) to a ubiquitous eukaryotic protein, t-complex polypeptide-1 (TCP1). In Saccharomyces cerevisiae, TCP1 is an essential protein that may play a part in mitotic spindle formation. We suggest that TF55 in archaebacteria and TCP1 in the eukaryotic cytosol are members of a new class of molecular chaperones.

Nucleotide sequence of mouse Tcp-1a cDNA

We have isolated complete cDNA clones encoding the mouse t-complex polypeptides 1A and 1B (TCP-1A and TCP-1B) from t-haplotype and wild-type (wt) mice, respectively. The complete nucleotide (nt) sequence of the Tcp-1a cDNA was determined. The Tcp-1a cDNA has an open reading frame (ORF) encoding a 60-kDa protein of 556 amino acids (aa). A comparison of nt sequences between the Tcp-1a and Tcp-1b cDNAs revealed that the 1786-bp regions upstream from their polyadenylation signals differed by 17 substitutions and that Tcp-1a had different polyadenylation sites from Tcp-1b. In these ORFs, 15 bp were substituted between the two alleles, occurring in 14 codons and resulting in eleven single-aa substitutions. Among these 15 substitutions, twelve were nonsynonymous (aa change) and three were synonymous (no aa change). The aa substitution in TCP-1 has occurred at least 20 times faster between t-haplotype and wt than between mouse and human or mouse and Drosophila.

The yeast homolog to mouse Tcp-1 affects microtubule-mediated processes

A Saccharomyces cerevisiae homolog to Drosophila melanogaster and mouse Tcp-1 encoding tailless complex polypeptide 1 (TCP1) has been identified, sequenced, and mapped. The mouse t complex has been under scrutiny for six decades because of its effects on embryogenesis and sperm differentiation and function. TCP1 is an essential gene in yeast cells and is located on chromosome 4R, linked to pet14. The TCP1-encoded proteins in yeast, Drosophila, and mouse cells share between 61 and 72% amino acid sequence identities, suggesting a primordial function for the TCP1 gene product. To assess function, we constructed a cold-impaired recessive mutation (tcp1-1) in the yeast gene. Cells carrying the tcp1-1 mutation grew linearly rather than exponentially at the restrictive temperature of 15 degrees C with a generation time of approximately 32 h in minimal medium. Both multinucleate and anucleate cells accumulated with time, suggesting that the linear growth kinetics may be explained by the generation of anucleate buds incapable of further cell division. In addition, the multinucleate and anucleate cells contained morphologically abnormal structures detected by anti-alpha-tubulin antibodies. The kinetics of appearance of these abnormalities suggest that they are a direct consequence of loss of function of the TCP1 protein and not a delayed, indirect consequence of cell death. We also observed that strains carrying tcp1-1 were hypersensitive to antimitotic compounds. Taken together, these observations imply that the TCP1 protein affects microtubule-mediated processes.

The yeast homolog to mouse Tcp-1 affects microtubule-mediated processes

A Saccharomyces cerevisiae homolog to Drosophila melanogaster and mouse Tcp-1 encoding tailless complex polypeptide 1 (TCP1) has been identified, sequenced, and mapped. The mouse t complex has been under scrutiny for six decades because of its effects on embryogenesis and sperm differentiation and function. TCP1 is an essential gene in yeast cells and is located on chromosome 4R, linked to pet14. The TCP1-encoded proteins in yeast, Drosophila, and mouse cells share between 61 and 72% amino acid sequence identities, suggesting a primordial function for the TCP1 gene product. To assess function, we constructed a cold-impaired recessive mutation (tcp1-1) in the yeast gene. Cells carrying the tcp1-1 mutation grew linearly rather than exponentially at the restrictive temperature of 15 degrees C with a generation time of approximately 32 h in minimal medium. Both multinucleate and anucleate cells accumulated with time, suggesting that the linear growth kinetics may be explained by the generation of anucleate buds incapable of further cell division. In addition, the multinucleate and anucleate cells contained morphologically abnormal structures detected by anti-alpha-tubulin antibodies. The kinetics of appearance of these abnormalities suggest that they are a direct consequence of loss of function of the TCP1 protein and not a delayed, indirect consequence of cell death. We also observed that strains carrying tcp1-1 were hypersensitive to antimitotic compounds. Taken together, these observations imply that the TCP1 protein affects microtubule-mediated processes.

Isolation and characterization of a cDNA clone corresponding to the mouse t-complex gene Tcp-1x

The mouse t complex on chromosome 17 is known to harbour many genes which have an important role in spermatogenesis. One of these, Tcp-1 has been cloned and shown to code for a protein probably essential for acrosome formation. During the isolation of a cDNA for Tcp-1 two other homologous sequences were recognized and described as Tcp-1x and Tcp-1y. In this paper we describe the isolation of a cDNA which has been shown by in situ hybridization to correspond to the Tcp-1x gene. Sequence analysis has confirmed that a 140 bp region of homology between Tcp-1 and Tcp-1x lies in the 3' portion of both genes. Northern blotting has revealed that the Tcp-1x gene is expressed abundantly in liver where two transcripts are detectable and hybrid selection shows that the gene codes for a 37 kDa protein. A search of the DNA database has failed to find any significant homology between Tcp-1x and any other sequences apart from Tcp-1.

Tcp-1 gene is not responsible for the maternal lethality effect of Thp mutation in mice

Embryos receiving either the Thp or the twLub2 mutation from their mother die during gestation. In contrast, heterozygous embryos receiving the mutant chromosome from the father are completely viable. In both Thp and twLub2 mutants, one chromosome 17 carries a deletion, which suggests the existence of a discrete maternal lethality effect locus (Tme). In the region whose deletion is responsible for the Thp or twLub2 mutation, a single gene has been cloned so far, the Tcp-1 gene. In the present study, we examined the expression of Tcp-1 gene in mutant embryos carrying either a maternal or a paternal Thp chromosome to test whether this gene could be involved in the Tme effect. We found that the levels of Tcp-1-specific transcripts were similar in both mutant embryos from reciprocal crosses and corresponded to half the levels found in the normal littermates. In addition, the paternal and maternal Tcp-1 alleles had identical methylation patterns. These results indicate that Tcp-1 is not responsible for the maternal lethality effect, and therefore is not located at the Tme locus.

Molecular Chaperones: The Plant Connection

Molecular chaperones are a family of unrelated proteins found in all types of cell. They mediate the correct assembly of other polypeptides, but are not components of the mature assembled structures. Chaperones function by binding specifically to interactive protein surfaces that are exposed transiently during many cellular processes and so prevent them from undergoing incorrect interactions that might produce nonfunctional structures. The concept of molecular chaperones originated largely from studies of the chloroplast enzyme rubisco, which fixes carbon dioxide in plant photosynthesis; the function of chaperones forces a rethinking of the principle of protein self-assembly.

Cloning of a Chinese hamster protein homologous to the mouse t-complex protein TCP-1: structural similarity to the ubiquitous 'chaperonin' family of heat-shock proteins

The complete cDNA sequence of a Chinese hamster ovary cell protein, homologous to a mouse t-complex protein, TCP-1, has been determined. The deduced amino acid sequences of the mouse/Chinese hamster TCP-1 proteins exhibit significant identity to the 60-65 kDa heat shock 'chaperonin' family of proteins present in prokaryotes and in the eukaryotic cell organelles.

Expression of three t-complex genes, Tcp-1, D17Leh117c3, and D17Leh66, in purified murine spermatogenic cell populations

Transmission ratio distortion (TRD) is a property of the complete t-haplotype which results in the preferential transmission of the t-haplotype chromosome from heterozygous t/+ males to the majority of the progeny. Available data suggest that in t/+ males, a dysfunction of the wild-type sperm within the female reproductive tract is responsible for the observed deviation from Mendelian segregation ratios. Genetically, Lyon has shown that multiple loci within the t-complex are required for maximum levels of TRD. These loci include multiple t-complex distorters (Tcds) which act upon a single t-complex responder (Tcr). Testis-expressed genes have been cloned which map to the same subregions of the t-complex as the Tcds and Tcr and are thus considered candidate genes for the products of these loci. To begin to understand how the products of these loci biochemically control TRD, the expression of three TRD-candidate genes (Tcp-1, D17Leh117c3, and D17Leh66) has been determined in populations of spermatocytes and differentiated spermatids purified to near homogeneity by unit gravity sedimentation. Fractions covering the entire gradient were analysed resulting in a more accurate picture of the precise timing of expression than previously reported. Transcription of all three genes was up-regulated in pachytene primary spermatocytes and persisted at stable levels through the haploid spermatid stages. Significantly, only levels of mRNA encoded by D17Leh66, the candidate gene for Tcr, increased from early round to elongating-stage spermatids. If this pattern of expression does, in fact, represent Tcr, these data provide the first direct evidence that wild-type and t-haplotype Tcr elements could be differentially expressed in haploid spermatids.

Post-meiotic gene expression

Evolutionary arguments and well-designed experiments (based on false premises, however) had suggested that post-meiotic gene expression did not occur in animals. The techniques of molecular genetics have now clearly demonstrated such genetic activity in mammalian testes. The current problem is to understand why some classes of genes, such as Zfy and many oncogenes, are expressed in this manner.

Identification of a Golgi-associated protein that undergoes mitosis dependent phosphorylation and relocation

By means of a monoclonal antibody (BH3), we have identified a 57-kD protein (p57) that in interphase is restricted largely to the perinuclear region of the cell. Double label immunofluorescence microscopy suggests localization of p57 to the Golgi complex and associated membranous structures. Protease protection experiments and chemical extractability indicate that p57 is a peripheral membrane protein exposed to the cytoplasm. p57 displays unique behavior during mitosis. At the end of G2 or in early prophase, p57 leaves the perinuclear region and accumulates very rapidly within the nucleus, at a time when the nuclear envelope is still intact and before nuclear lamina disassembly. This relocation of p57 coincides with its hyperphosphorylation on serine and threonine residues. After nuclear envelope breakdown p57 becomes uniformly distributed throughout the mitotic cytoplasm until in late telophase when it returns to its perinuclear location and is once again excluded from the nucleus. The behavior of p57 during mitosis suggests that it may play a role in the cellular reorganization evident during mitotic prophase.