Identification and Characterization of Agv1, a Pre-Metazoan Arf GAP: A Dissertation

Abstract

Human immunodeficiency virus type 1 (HIV-1) is a member of the lentivirus subfamily of retroviruses. HIV-1 expresses multiple genes from a single provirus by alternative splicing. Early in viral expression, fully spliced 2-kb viral RNA is exported from the nucleus and encodes the viral regulatory protein, Rev, which is essential for nuclear transport of partially spliced and unspliced genomic-length RNA. Rev binds to an RNA structural element called the Rev response element (RRE) and mediates nuclear export through the leucine-rich nuclear export signal (NES) pathway. The human Rev Interacting Protein (hRIP) interacts specifically with the Rev NES. Rev NES mutants that are unable to export Rev-dependent RNAs are also unable to bind to hRIP. The hRIP cDNA encodes a 562 amino acid protein containing an N-terminal zinc finger with homology to Arf GAP domains, a central serine and threonine rich region, and C-terminal phenylalanine-glycine (FG) repeats characteristic of nucleoporins. To identify an hRIP ortholog in a genetically tractable organism, we performed database searches using the N-terminal zinc finger of hRIP. Using this approach, we identified a novel gene in Schizosaccharomyces pombe. Alignment of the entire reading frame of the putative ortholog with hRIP indicates similarity with the serine/threonine rich region and with the FG repeats, suggesting that S. pombecould be a good model system to study the cellular function of hRIP. We find that the S. pombe ORF is an essential gene, which encodes a 483 amino acid protein that is also able to interact with the NES of HIV-1 Rev. Based on being an essential gene, and the presence of a putative Arf GAP domain, the ORF was named an Arf GAP essential for viability, agv1+. We show that Agv1 is not directly involved in the nuclear export of poly(A+) RNA or 5S rRNA, nuclear export of leucine-rich NES-containing proteins, or nuclear import of nuclear localization signal (NLS)-containing proteins. However, Agv1 does appear to play a role in the cytoplasmic localization of 5S rRNA. We demonstrate that loss of Agv1 alters the localization of endoplasmic reticulum (ER) membrane and Golgi membrane resident proteins, accumulates intracellular membrane, and blocks processing of carboxypeptidase Y. Furthermore, the S. cerevisiae ADP-ribosylation factor (Arf) GTPase activating protein (GAP) Glo3, but not a catalytically inactive Glo3 mutant [R59K], is able to partially compensate for the loss of Agv1 function in temperature sensitive strains, indicating that Agv1 is an S. pombe Arf GAP with some functional features similar to S. cerevisiae Glo3.