May promote appropriate targeting of ribosome-nascent polypeptide complexes (By similarity). Required for correct localization of the osk/oskar protein to the posterior pole during embryonic development. The osk protein directs the recruitment of molecules responsible for posterior body patterning and germline formation in the embryo. Part of the nascent polypeptide-associated complex (NAC), consisting of Nac-alpha and bicaudal (bic). Belongs to the NAC-alpha family.