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

There are several precedents for a transmembrane segment serving as a sorting determinant. The first of three transmembrane segments of a viral protein has been shown to be the topogenic determinant for sorting to the cis-Golgi membrane (Machamer and Rose, 1987; Swift and Machamer, 1991) . Moreover, the single transmembrane segments of two glycosyltransferases in the trans-Golgi have been identified as sorting determinants (Munro, 1991; Nilsson et al., 1991; Teasdale et al., 1992; Wong et al., 1992) . And, the single transmembrane segment of gp210 serves as principal determinant for sorting to the pore membrane domain of the nuclear envelope (Wozniak and Blobel, 1992) . In none of these cases has the precise mechanism by which the transmembrane segment functions in sorting been established.