The first membrane spanning region of the lamin B receptor is sufficient for sorting to the inner nuclear membrane

The specific structural features of the first transmembrane helix region of LBR that mediate specific interaction with another transmembrane helix remain to be determined. The hydrophobic transmembrane segment is free of charged residues eliminating ion pair formation as a means for interhelical association. Interestingly, there is a cysteine (Cys 226) at the cisternal end of the transmembrane segment (see Fig. 6 B). Being exposed to an oxidizing environment this cysteine could potentially form a disulfide bond with the similarly located cysteine of the first transmembrane segment of a second LBR (or another protein). An example of this has been described recently, where disulfide linked dimerization of the g" chain subunit of the T cell antigen receptor was shown to be mediated by a cysteine located at the lumenal end of the transmembrane domain (Rutledge et al., 1992) . We are currently investigating the role of Cys 226 in LBR dimerization.