CHARACTERIZATION OF P18, A COMPONENT OF THE LAMIN-B RECEPTOR COMPLEX AND A NEW INTEGRAL MEMBRANE-PROTEIN OF THE AVIAN ERYTHROCYTE NUCLEAR-ENVELOPE

Employing avian erythrocytes, we have previously isolated a multimeric complex consisting of the lamin B receptor (LBR, or p58), the nuclear lamins, an LBR-specific kinase, a 34-kDa protein, and an 18-kDa polyp eptide termed p18. As the LBR kinase and the 34-kDa component have bee n recently characterized, we now proceed in the characterization of p1 8. We show here that p18 is an integral membrane protein specific to t he erythrocyte nuclear envelope which binds to LBR and B-type lamins. NH2-terminal sequencing indicates that p18 is distinct from other nucl ear envelope components, but has similarity to the mitochondrial isoqu inoline-binding protein. In situ analysis by immunoelectron microscopy and examination of digitonin-permeabilized cells by indirect immunofl uorescence show that p18, unlike LBR and other lamin-binding proteins, is equally distributed between the inner and outer nuclear membrane. Furthermore, cycloheximide inhibition experiments reveal that the frac tion of p18 that resides in the outer nuclear membrane does not repres ent nascent chains en route to the inner nuclear membrane, but rather material in equilibrium with the p18 that partitions with the inner nu clear membrane. The paradigm of p18 suggests that transmembrane comple xes formed by the nuclear lamins and LBR provide potential docking sit es for integral membrane proteins of the nuclear envelope that equilib rate between the rough endoplasmic reticulum and the inner nuclear mem brane.