A novel cellular protein, Abl-interactor-1 (Abi-1), which specifically interacts with the carboxy-terminal region of Abl oncoproteins, has been identified in a mouse leukemia cell line. The protein exhibits sequence similarity to homeotic genes, contains several polyproline stretches, and includes a src homology 3 {SH3) domain at its very carboxyl terminus that is required for binding to Abl proteins. The abi-1 gene has been mapped to mouse chromosome 2 and is genetically closely linked to the c-abi locus. The gene is widely expressed in the mouse, with highest levels of mRNA found in the bone marrow, spleen, brain, and testes. The Abi-1 protein coimmunoprecipitates with v-Abl and serves as a substrate for kinase activity. When overexpressed in NIH-3T3 cells, abi-1 potently suppresses the transforming activity of Abelson leukemia virus expressing the full-length pi60^'"*^ kinase but does not affect the transforming activity of viruses expressing a truncated pPO^"*'' or \-src kinases. We suggest that the Abi-1 protein may serve as a regulator of Abl function in transformation or in signal transduction.
The capsid domain (CA) of the retroviral Gag protein is a major constituent of the virion core. To examine the role of this protein in M-MuLV morphogenesis and replication, a series of substitution mutations affecting the central region of CA were introduced into an infectious proviral DNA. The altered DNAs were introduced into cells, and the resulting lines were analyzed for production of infectious virions. Only one of the replication defective mutants analyzed was blocked in virion assembly. The remaining mutant DNAs induced the formation and release of particles containing genomic RNA and polymerase protein. The reverse transcriptase associated with these mutant virions was capable of transcribing both minus strand strong stop and extended DNA products using the endogenous genomic RNA as template in vitro. Upon infection of fresh cells, however, no viral DNA synthesis could be detected either by Southern analysis or by an RNase protection assay developed specifically to detect intermediate products of reverse transcription. The results indicate that the bulk of the CA mutants are blocked before reverse transcription of the viral genome and suggest an important role for the capsid protein in an early stage of viral replication.
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