Truncated Isoforms of Kap60 Facilitate Trafficking of Heh2 to the Nuclear Envelope

Isoforms of importin-alpha have been identified in insect and human cells, and cross-linking experiments suggest that at least one isoform in each species participates in the targeting of integral membrane proteins to the inner nuclear membrane (INM). To directly test this hypothesis, an assay was developed using Saccharomyces cerevisiae. The data show that internal promoters are present within KAP60, and the nested transcripts are translated into three isoforms: Kap60-44, Kap60-30 and Kap60-10. In the absence of the isoforms, the INM protein Heh2-green fluorescent protein (GFP) localized to cytoplasmic membranes, whereas its wild-type localization at the nuclear periphery was restored when the Kap60-44 isoform was reintroduced. An INM-sorting sequence has been identified that cross-links with the isoform of importin-alpha that directs trafficking toward the nuclear envelope (NE). When this sequence in HEH2 was mutated, Heh2 was again localized to cytoplasmic membranes. Thus, this report provides the first evidence that isoforms of Kap60 exist in yeast, and these isoforms participate in the molecular sorting and enrichment of INM proteins to the NE. Herein, we provide additional support for the hypothesis that trafficking of INM proteins to the NE is a regulated process.