A putative zinc-binding protein on lampbrush chromosome loops.

We used mAb A33/22, which recognizes a nuclear protein on the loops of amphibian lampbrush chromosomes, to select cDNA clone PwA33 from an expression library of the newt Pleurodeles waltl. A myc-tagged transcript of clone PwA33 was injected into Pleurodeles oocytes. The translation product localized in the germinal vesicle (GV) and was distributed on the lampbrush loops in a pattern identical to that of the endogenous protein. PwA33 encodes a 71 kDa protein with three distinct domains: a region rich in Cys/His residues that may form zinc fingers, a coiled-coil domain with potential for dimerization and a third 'rfp-like' domain that is shared by several other nuclear proteins. The putative zinc fingers and the coiled-coil domain resemble features in known nucleic acid-binding regulatory proteins. These structures, coupled with a distinctive pattern of expression in embryonic tissues, suggest that A33 may function as a regulatory protein during early development. It is unlikely that the large store of A33 in the GV is bound to DNA. Instead, its association with the nascent transcripts on the lampbrush chromosome loops suggests a role in pre-mRNA synthesis or processing.