Proteins establish and maintain a distinct intracellular localization by means of targeting, retention, and retrieval signals, ensuring most proteins reside predominantly in one cellular location. The enzymes involved in the maturation of lamin A present a challenge to this paradigm. Lamin A is first synthesized as a 74-kDa precursor, prelamin A, with a C-terminal CaaX motif and undergoes a series of posttranslational modifications including CaaX processing (farnesylation, aaX cleavage and carboxylmethylation), followed by endoproteolytic cleavage by Zmpste24. Failure to cleave prelamin A results in progeria and related premature aging disorders. Evidence suggests prelamin A is imported directly into the nucleus where it is processed. Paradoxically, the processing enzymes have been shown to reside in the cytosol (farnesyltransferase), or are ER membrane proteins (Zmpste24, Rce1, and Icmt) with their active sites facing the cytosol. Here we have reexamined the cellular site of prelamin A processing, and show that the mammalian and yeast processing enzymes Zmpste24 and Icmt exhibit a dual localization to the inner nuclear membrane, as well as the ER membrane. Our findings reveal the nucleus to be a physiologically relevant location for CaaX processing, and provide insight into the biology of a protein at the center of devastating progeroid diseases. INTRODUCTIONThe nuclear envelope consists of an inner and outer membrane. The outer nuclear membrane (ONM) is continuous with the endoplasmic reticulum (ER) and connects with the inner nuclear membrane (INM) at the nuclear pore membrane (POM; Lusk et al., 2007). Integral membrane proteins that reside in the INM are initially inserted into the endoplasmic reticulum (ER) membrane and move via the POM to the INM where they maintain a concentrated localization by binding to chromatin or tethering to the nuclear lamina (Powell and Burke, 1990;Soullam and Worman, 1993;Holmer and Worman, 2001;Ohba et al., 2004;Lusk et al., 2007;Schirmer and Foisner, 2007). Until recently, conventional wisdom held that the localization of proteins to either the ER membrane or the INM is mutually exclusive. For example, proteins that are found within the INM, such as LAP1, are not generally found throughout the ER/ONM; likewise, ER proteins such as HMG-CoA reductase are strictly localized to the ER (Wright et al., 1988;Powell and Burke, 1990;Deng and Hochstrasser, 2006). It was recently shown that a notable exception to this paradigm is the yeast E3 ubiquitin ligase, Doa10p. Doa10p was long known to reside in the ER membrane (Swanson et al., 2001;Kreft et al., 2006). However, recent data indicate that Doa10p exhibits dual steady-state localizations, residing and functioning in both the ER membrane where it mediates ER-associated degradation of membrane and secretory proteins, and in the INM where it mediates degradation of the nuclear transcription factor MAT␣2 (Deng and Hochstrasser, 2006).The present study documents another exception to the mutual exclusivity of ER membrane and INM localizati...

The Saccharomyces cerevisiae mating pheromone a-factor provides a paradigm for understanding the biogenesis of prenylated fungal pheromones. The biogenesis of a-factor involves multiple steps: (i) C-terminal CAAX modification (where C is cysteine, A is aliphatic, and X is any residue) which includes prenylation, proteolysis, and carboxymethylation (by Ram1p/Ram2p, Ste24p or Rce1p, and Ste14p, respectively); (ii) N-terminal processing, involving two sequential proteolytic cleavages (by Ste24p and Axl1p); and (iii) nonclassical export (by Ste6p). Once exported, mature a-factor interacts with the Ste3p receptor on MAT␣ cells to stimulate mating. The a-factor biogenesis machinery is well defined, as is the CAAX motif that directs C-terminal modification; however, very little is known about the sequence determinants within a-factor required for N-terminal processing, activity, and export. Here we generated a large collection of a-factor mutants and identified residues critical for the N-terminal processing steps mediated by Ste24p and Axl1p. We also identified mutants that fail to support mating but do not affect biogenesis or export, suggesting a defective interaction with the Ste3p receptor. Mutants significantly impaired in export were also found, providing evidence that the Ste6p transporter recognizes sequence determinants as well as CAAX modifications. We also performed a phenotypic analysis of the entire set of isogenic a-factor biogenesis machinery mutants, which revealed information about the dependency of biogenesis steps upon one another, and demonstrated that export by Ste6p requires the completion of all processing events. Overall, this comprehensive analysis will provide a useful framework for the study of other fungal pheromones, as well as prenylated metazoan proteins involved in development and aging.Most fungi secrete pheromones that play important signaling roles in mating to stimulate cell and/or nuclear fusion. In Saccharomyces cerevisiae and other ascomycetes, the mating pheromones produced by the two mating types differ, in that one is an unmodified peptide, while the other is a prenylated and carboxymethylated peptide (i.e., ␣-factor and a-factor, respectively) (3,7,21,24,25,41,49,59,61,70). Basidomycetes, which can have two or even more mating types, encode exclusively the latter class of modified peptide pheromones (8,13,18,26,27,36,47,56,60). These pheromones are synthesized as precursors terminating in a C-terminal CAAX motif (where C is cysteine, A is usually aliphatic, and X is any residue). The S. cerevisiae pheromone a-factor is the best-characterized example of this class of fungal pheromones and provides a paradigm for understanding their biogenesis and secretion (14,20). Studies of a-factor have also yielded critical insights into the processing and properties of eukaryotic CAAX proteins in general, which include Ras and other small GTP-binding proteins, the ␥-subunits of heterotrimeric G proteins, and the nuclear lamins A and B (30,68,69).Components of the machinery required for th...

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Single-strand annealing (SSA) is an important homologous recombination mechanism that repairs DNA double strand breaks (DSBs) occurring between closely spaced repeat sequences. During SSA, the DSB is acted upon by exonucleases to reveal complementary sequences that anneal and are then repaired through tail clipping, DNA synthesis, and ligation steps. In baker's yeast, the Msh DNA mismatch recognition complex and the Sgs1 helicase act to suppress SSA between divergent sequences by binding to mismatches present in heteroduplex DNA intermediates and triggering a DNA unwinding mechanism known as heteroduplex rejection. Using baker's yeast as a model, we have identified new factors and regulatory steps in heteroduplex rejection during SSA. First we showed that Top3-Rmi1, a topoisomerase complex that interacts with Sgs1, is required for heteroduplex rejection. Second, we found that the replication processivity clamp proliferating cell nuclear antigen (PCNA) is dispensable for heteroduplex rejection, but is important for repairing mismatches formed during SSA. Third, we showed that modest overexpression of Msh6 results in a significant increase in heteroduplex rejection; this increase is due to a compromise in Msh2-Msh3 function required for the clipping of 39 tails. Thus 39 tail clipping during SSA is a critical regulatory step in the repair vs. rejection decision; rejection is favored before the 39 tails are clipped. Unexpectedly, Msh6 overexpression, through interactions with PCNA, disrupted heteroduplex rejection between divergent sequences in another recombination substrate. These observations illustrate the delicate balance that exists between repair and replication factors to optimize genome stability.