KH DOMAIN INTEGRITY IS REQUIRED FOR WILD-TYPE LOCALIZATION OF SAM68

The protein Sam68 (Src-associated in mitosis, 68 kDa) has been found t o bind to SH2 and to SH3 domain-containing proteins and to RNA, Althou gh its protein-protein interactions implicate Sam68 in cell signaling the significance of its RNA binding remains obscure. in most cells, Sa m68 shows diffuse nucleoplasmic staining, Upon treatment with transcri ption inhibitors, however, Sam68 localized into punctate nuclear struc tures. Mutant forms of mouse Sam68 were overexpressed in human cells t o test the importance of the KH domain, which is required for RNA bind ing in the intracellular localization of Sam68. A small deletion withi n the KH domain (Delta 206-218) or point mutation I184N had no effect upon the localization of overexpressed Sam68. Sam68 that contained a d eletion of the entire KH domain (Delta KH, Delta 157-256) or poind; mu tation G178E, however, localized to distinct nuclear spats. Furthermor e, Delta KH Sam68, unlike wild-type Sam68 and several other mutant Sam 68 proteins, did mot relocalize upon poliovirus infection and caused t he normally cytoplasmic viral polymerase to localize to the nuclear sp ots. Thus both ongoing transcription and an intact KH domain are cruci al determinants of the dynamic intracellular localization of Sam68. (C ) 1998 Academic Press.