GAPIII A NEW BRAIN-ENRICHED MEMBER OF THE GTPASE-ACTIVATING PROTEIN FAMILY

Ras GTPase-activating proteins (GAPs) are negative regulators of ras, which controls proliferation and differentiation in many cells. Ras GA Ps have been found in a variety of species from yeast to mammals. We d escribe here a newly identified mammalian GAP, GapIII, which was obtai ned by differential screening of a rat oligodendrocyte cDNA library. G apIII putatively encodes a 834 amino acid protein with a predicted mol ecular weight of 96 kDa, which contains a consensus GAP-related domain (GRD). The protein encoded by this cDNA has high homology with Gap1(m ), which was recently identified as a putative mammalian homolog of Dr osophila Gap1. These proteins contain three structural domains, an N-t erminal calcium-dependent phospholipid binding domain, GRD, and a C-te rminal PH/Btk domain. Because of the sequence homology and the structu ral similarities of this protein with Gap1(m), we hypothesize that Gap III and Gap1(m) may be members of a mammalian GAP gene family, separat e from p120GAP, neurofibromin (NF1), and IQGAP. To confirm the GapIII protein activity, constructs containing different GapIII-GRD domains w ere transformed into ira1 mutant yeast to determine their relative abi lity to replace IRA1 functionally. Constructs that contained essential ly the full-length protein (all three domains), the GRD alone, or the GRD plus PH/Btk domain suppressed heat shock sensitivity of ira1, wher eas constructs that contained the GRD with part of the PH/Btk domain h ad only a weak ability to suppress heat shock sensitivity. These resul ts suggest that the GapIII GRD itself is sufficient to down-regulate r as proteins in yeast. Expression of GapIII mRNA (4.2 kb) was examined by Northern analysis and in situ hybridization. This mRNA was expresse d at highest levels in the brain, where its expression increased with development. Lower levels of the mRNA were expressed in the spleen and lung. Among neural cells, GapIII mRNA was expressed in neurons and ol igodendrocytes, but not in astrocytes. Interestingly, the expression p attern in brain is reminiscent of type 1 NF1 expression reported by Gu tmann et al. (Cell Growth Differ in press, 1995). We propose that in a ddition to p120GAP and neurofibromin, the GapIII/Gap1(m) family may be important for modulating ras activity in neurons and oligodendrocytes during normal brain development and in particular in the adult brain. (C) 1995 Wiley-Liss, Inc.