Encoding Protein Isoforms Research Articles

Molecular and genetic analysis of the mouse homolog of the Drosophila suppressor of position-effect variegation 3-9 gene.

The Drosophila melanogaster gene suppressor of position-effect variegation 3-9 [Su(var)3-9] encodes a component of heterochromatin with a chromodomain and a SET domain. Here, we describe the cloning of a mouse homolog called Suv39hl and describe the genomic organization, pattern of expression, and genetic map position. The genomic locus is approximately 10 kb and consists of five exons. The first two exons, 1a and 1b, are alternative first exons and are followed by three common exons. Two mRNAs, encompassing exon 1a or 1b, encode protein isoforms with distinct amino termini, but which are otherwise identical, including the chromodomain and SET domain. Interestingly, only one of the isoforms contains a putative nuclear localization signal. Consistent with other genes encoding proteins associated with chromatin structure, Suv39hl is expressed in a widespread manner. Interspecific backcross mapping localized Suv39hl near tattered (Td) and scurfy (sf) on the proximal X Chromosome (Chr). However, analysis of Td/Y and sf/Y mutant stocks indicated that Suv39hl is not responsible for either mutant phenotype.

WT1: what has the last decade told us?

When positionally cloned in late 1989, it was anticipated that mutations within the Wilms' tumour suppressor gene (WT1) would prove responsible for this common solid kidney cancer of childhood. Characterisation of the WT1 expression pattern and of the structure of the encoded protein isoforms and their mode of action has now spanned almost a decade. WT1 proteins act as nucleic acid-binding zinc finger-containing transcription factors involved in both transactivation and repression. These activities are facilitated and constrained by interactions with other proteins. Expression analyses and knockout mice indicate that WT1 protein plays a critical role in normal kidney and gonad development. Specific constitutional WT1 mutations results in several urogenital anomaly syndromes. While only 10% of sporadic Wilms' tumours do display WT1 mutation, WT1 is mutated in other cancers, including acute myeloid leukaemia. Much is still to be determined in WT1 biology. The next decade will see at least three streams of attention. The first two, elucidation of the role of WT1 in RNA metabolism and the characterisation of further protein partners, may together explain the distinct tissue-specific functions of WT1. Finally, further research into the role of WT1 in haematopoiesis will improve our understanding of WT1 in leukaemia.

A single mouse gene encodes the mitochondrial transcription factor A and a testis–specific nuclear HMG-box protein

Mitochondrial transcription factor A (mtTFA) is a key regulator of mammalian mitochondrial DNA transcription. We report here that a testis-specific isoform of mouse mtTFA lacks the mitochondrial targeting sequence and is present in the nucleus of spermatocytes and elongating spermatids, thus representing the first reported mammalian gene encoding protein isoforms targeted for the mitochondria or the nucleus. The presence of the mitochondrial transcriptional activator in the nucleus raises the possibility of a role for this protein in both genetic systems. Mutations in the nuclear mtTFA gene may therefore exhibit phenotypic consequences due to altered function in either or both genetic compartments.

Regulation of Human Insulin Receptor RNA Splicing in HepG2 Cells: Effects of Glucocorticoid and Low Glucose Concentration

Human insulin receptor RNA is expressed in two alternatively spliced forms (Ex. 11− and Ex. 11+) encoding protein isoforms with discrete functional differences. In vivo , the splicing varies between tissues and changes concomitantly with the control of glucose homeostasis. Recently, dexamethasone has been described to increase the relative expression of Ex. 11+ RNA molecules. In this study, we confirm that dexamethasone dose-dependently increases inclusion of exon 11, and demonstrate that low concentrations of glucose decrease inclusion. No effect was observed of either insulin or IGF1. These results suggest that hormonal as well as metabolic factors regulate the splicing of insulin receptor RNA in HepG2 cells.

Protein isoforms encoded by the pX region of human T-cell leukemia/lymphotropic virus type I.

The pX region of the human T-cell leukemia/lymphotropic virus type I (HTLV-I) contains at least four open reading frames (orfI-orfIV). orf III and orf IV encode the regulatory HTLV-I proteins Rex and Tax, which together modulate viral expression, and the p21rex protein of unknown function. By using the reverse transcriptase and polymerase chain reaction techniques on the RNA of an HTLV-I-infected cell culture, we uncovered the existence of alternatively spliced mRNAs generated through the use of three splice acceptor sites. These mRNAs encoded protein isoforms derived from the HTLV-I orf I (p12I) and orf II (p13II and p30II). An additional acceptor splice site, used in the processing of the env and tax/rex mRNAs and a singly spliced mRNA for the p21rex protein, was also identified. All of these HTLV-I mRNAs were also detected in freshly isolated cells from HTLV-I-infected individuals. Thus HTLV-I, like the human immunodeficiency virus type 1, has developed fine posttranscriptional mechanisms to increase the complexity of its genome.