Initial characterization of the nascent polypeptide‐associated complex in yeast

Abstract

The three subunits of the nascent polypeptide-associated complex ( , 1 , 3 ) in Saccharomyces cerevisiae are encoded by three genes (EGD2, EGD1, BTT1). We found the complex bound to ribosomes via the -subunits in a salt-sensitive manner, in close proximity to nascent polypeptides. Estimation of the molecular weight of the complex of wild-type cells and cells lacking one or two subunits revealed that the composition of the complex is variable and that as yet unknown proteins might be included. Regardless of the variability, a certain balance of the subunits has to be maintained: the deletion of one subunit causes downregulation of the remaining subunits at physiological growth temperature. Cells lacking both -subunits are unable to grow at 37 C, most likely due to a toxic effect of the -subunit. Based on in vitro experiments, it has been proposed that the function of mammalian nascent-polypeptide associated complexes (NAC) is to prevent inappropriate targeting of non-secretory nascent polypeptides. In vivo, however, the lack of NAC does not cause secretion of signal-less invertase in yeast. This result and the lack of a drastic phenotype of cells missing one, two or three subunits at optimal conditions (28 C, YPD-medium) suggest either the existence of a substitute for NAC or that cells tolerate or 'repair' the damage caused by the absence of NAC.

αβ′‐NAC cooperates with Sam37 to mediate early stages of mitochondrial protein import

“…In our genetic background, even on synthetic media at 37°C, we did not detect any growth differences in contrast to what was reported before at higher temperature (42°C; Fig. 1) [23]. Then, after generating the two independent sets of double DaDx and triple DbDb 0 Dx mutants (where Dx represents any of the 4844 viable deletion mutants from the BY4741 yeast deletion collection), we were able to identify 131 and 76 aggravating genetic interactions of genes with the a-NAC and bb 0 -NAC components, respectively (Tables S1 and S2).…”

“…First, we verified that each of the bait strains (Da and DbDb 0 ) showed no growth defect on fermentable or non-fermentable media as previously reported [23,25,34,35] (Fig. 1).…”

“…Such is the case for model organisms like nematodes [28], fruit flies [29] and mice [30]. However, it has been observed that yeast strains lacking all genes encoding NAC subunits show a wild-type phenotype under standard laboratory conditions [23]. We thus hypothesized the existence of at least one additional component in S. cerevisiae, which is functionally redundant to NAC and assists protein import into mitochondria.…”

“…Thus, there are two possible NAC heterodimers in yeast: ab and ab 0 . It is known that, under standard laboratory conditions, b is preferentially expressed and that its level of expression is similar to that of the a subunit; hence it has been suggested that ab-NAC is the complex performing the reported NAC functions [23]. It has been previously observed that yeasts mutants lacking ab-NAC subunits show defects in the import of mitochondrial matrix proteins such as Mdh1, Fum1 and Mmf1 as well as fusion proteins [24].…”

OST 1‐mediated BTF 3L phosphorylation positively regulates CBF s during plant cold responses

“…), fruit flies (Drosophila melanogaster), and Caenorhabditis elegans, suggesting that NAC is essential for growth and development (Deng & Behringer, 1995;Markesich et al, 2000). However, the loss of NAC does not result in obvious defects in yeast (Reimann et al, 1999). Rather, when both NAC and ribosome-associated HSP70 homologs are absent, substantial growth defects are observed, suggesting that NAC may be connected with the second ribosome-associated chaperone system (Koplin et al, 2010).…”