Haemin protects Met-tRNAf binding activity of isolated reticulocyte ribosomes from inactivation by protein kinase

Evidence that endoplasmic reticulum (ER)-associated degradation of cystic fibrosis transmembrane conductance regulator is linked to retrograde translocation from the ER membrane

The ubiquitin-proteasome pathway has been implicated in the degradation of newly synthesized, misfolded and unassembled proteins in the endoplasmic reticulum (ER). Using a cell-free reticulocyte lysate system we have examined the relationship between biosynthesis and ER-associated degradation of the cystic fibrosis transmembrane conductance regulator (CFTR), a polytopic protein with 12 predicted transmembrane segments. Our results provide direct evidence that full-length, glycosylated and membrane-integrated CFTR is a substrate for degradation and that degradation involves polyubiquitination and cytosolic proteolytic activity. CFTR ubiquitination was both temperature- and ATP-dependent. Degradation was significantly inhibited by EDTA, apyrase, and the proteasome inhibitors hemin and MG132. Degradation was inhibited to a lesser extent by clasto-lactacystin beta-lactone, ALLN, and Nalpha-tosyl-L-phenylalanine chloromethyl ketone and was relatively unaffected by lactacystin and N-tosyl lysyl chloromethyl ketone. In the presence of hemin, polyubiquitinated CFTR remained tightly associated with ER microsomes. However, membrane-bound ubiquitinated CFTR could be subsequently degraded into trichloroacetic acid-soluble fragments upon incubation in hemin-free, ATP-containing lysate. Thus ER-associated degradation of CFTR occurs via a membrane-bound, rather than cytosolic, intermediate and likely involves recruitment of degradation machinery to the ER membrane. Our data suggest a model in which the degradation of polytopic proteins such as CFTR is coupled to retrograde translocation and removal of the polypeptide from the lipid bilayer.

The effect of hemin, adenosine 3',5'-monophosphate and phosphorylated sugars on Met-tRNAfMet deacylase activity in rabbit reticulocyte lysates

The effect of hemin, phosphorylated sugars, adenosine 3',5'-monophosphate (cyclic AMP) and a number of purines on a specific initiator tRNA deacylase activity in rabbit reticulocytes has been investigated. In the concentration range established to be optimal for maximal stimulation of translation (5.5-30.0 microM), hemin produces a 20-82% inhibition of Met-tRNAfMet deacylation. In contrast, all phosphorylated sugars tested, with the exception of fructose 1,6-diphosphate, are without effect. High concentrations of cyclic AMP (2-4 mM) also significantly inhibit the deacylase activity. The role of hemin and Met-tRNAfMet deacylase in the control of peptide initiation are discussed.

Stimulation of protein synthesis and Met-tRNAf binding by phosphorylated sugars: studies on their mechanism of action

It has been previously reported by J. R. Lenz et al. [(1978) Biochemistry 17, 80--87] that certain phosphorylated sugars stimulate protein synthesis in extracts of mammalian cells. This effect was found to be due to a stimulation of Met-tRNAf binding to 40S ribosomal subunits, both in whole extracts and with isolated ribosomes. However, formation of a ternary complex of Met-tRNAf, initiation factor eIF-2, and GTP was not stimulated. It was also shown that the stimulation is not due solely to metabolism of the sugars. The present communication further characterizes the stimulatory effect of the sugars. They were found to prevent the inactivation of ribosomes that occurs during protein synthesis incubations. The sugars were also found to inhibit cAMP-dependent protein kinases noncompetitively. However, they stimulate Met-tRNAf binding to 40S ribosomal subunits even under conditions in which an inhibition of protein kinase has no effect. Although it has bot been possible to demonstrate a direct association of the sugars with the 40S initiation complex, the evidence suggests that their effect is mediated by an interaction with one of the components involved in the formation of this complex.