Purification and Characterization of Hen Oviduct Nα-Acetyltransferase

Structure of the human signal peptidase complex reveals the determinants for signal peptide cleavage

The signal peptidase complex (SPC) is an essential membrane complex in the endoplasmic reticulum (ER), where it removes signal peptides (SPs) from a large variety of secretory pre-proteins with exquisite specificity. Although the determinants of this process have been established empirically, the molecular details of SP recognition and removal remain elusive. Here, we show that the human SPC exists in two functional paralogs with distinct proteolytic subunits. We determined the atomic structures of both paralogs using electron cryo-microscopy and structural proteomics. The active site is formed by a catalytic triad and abuts the ER membrane, where a transmembrane window collectively formed by all subunits locally thins the bilayer. Molecular dynamics simulations indicate that this unique architecture generates specificity for SPs based on the length of their hydrophobic segments.

N-terminal protein modifications: Bringing back into play the ribosome

N-terminal protein modifications correspond to the first modifications which in principle any protein may undergo, before translation is completed by the ribosome. This class of essential modifications can have different nature or function and be catalyzed by a variety of dedicated enzymes. Here, we review the current state of the major N-terminal co-translational modifications, with a particular emphasis to their catalysts, which belong to metalloprotease and acyltransferase clans. The earliest of these modifications corresponds to the N-terminal methionine excision, an ubiquitous and essential process leading to the removal of the first methionine. N-alpha acetylation occurs also in all Kingdoms although its extent appears to be significantly increased in higher eukaryotes. Finally, N-myristoylation is a crucial pathway existing only in eukaryotes. Recent studies dealing on how some of these co-translational modifiers might work in close vicinity of the ribosome is starting to provide new information on when these modifications exactly take place on the elongating nascent chain and the interplay with other ribosome biogenesis factors taking in charge the nascent chains. Here a comprehensive overview of the recent advances in the field of N-terminal protein modifications is given.

Evidence for arylamine N-acetyltransferase activity in the fungi Candida albicans

N-acetyltransferase activities were determined in Candida albicans, which is a member of the normal flora of the mucous membranes in the respiratory, gastrointestinal and female genital tract. The N-acetylation of 2-aminofluorene and p-aminobenzoic acid by the N-acetyltransferase from Candida albicans was determined using high pressure liquid chromatography. The activities (mean +/- S.D.) of N-acetyltransferase from Candida albicans cytosols were 1.06 +/- 0.01 nmol/min per mg protein for the acetylation of 2-aminofluorene substrate, and not detectable levels of acetyl-p-aminobenzoic acid for the acetylation of p-aminobenzoic acid. The apparent kinetic constants Km and Vmax values were 0.17 +/- 0.06 mM and 1.43 +/- 0.42 nmol/min per mg protein, respectively, for 2-aminofluorene substrate. The optimum pH value for the enzyme activity was 8.0. The optimal temperature for the enzyme activity is 40 degrees C for 2-aminofluorene substrate. Among a series of divalent cations and salts, Fe2+, SCN-, I-, and NH4+ were demonstrated to be the most potent inhibitors. The N-acetyltransferase activity was inhibited by iodoacetamide: at 0.25 mM iodoacetamide, activity was reduced 50% and 1.0 mM iodoacetamide inhibited activity more than 90%. This is the first demonstration of acetyl CoA arylamine N-acetyltransferase activity in the yeast-like fungus Candida albicans.