Initial characterization of the nascent polypeptide-associated complex in yeast

The three subunits of the nascent polypeptide-associated complex (alpha, be ta(1), beta(3)) in Saccharomyces cerevisiae are encoded by three genes (EGD 2, EGD1, BTT1). We found the complex bound to ribosomes via the beta-subuni ts in a salt-sensitive manner, in close proximity to nascent polypeptides. Estimation of the molecular weight of the complex of wild-type cells and ce lls lacking one or two subunits revealed that the composition of the comple x is variable and that as yet unknown proteins might be included. Regardles s of the variability, a certain balance of the subunits has to be maintaine d: the deletion of one subunit causes downregulation of the remaining subun its at physiological growth temperature. Cells lacking both beta-subunits a re unable to grow at 37 degrees C, most likely due to a toxic effect of the alpha-subunit. Based on in vitro experiments, it has been proposed that th e function of mammalian nascent-polypeptide associated complexes (NAC) is t o prevent inappropriate targeting of non-secretory nascent polypeptides, In vivo, however, the lack of NAC does not cause secretion of signal-less inv ertase in yeast. This result and the lack of a drastic phenotype of cells m issing one, two or three subunits at optimal conditions (28 degrees 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. Copyright (C) 1999 John Wiley & Sons, Ltd.