No abstract

Digestion of the signal recognition particle (SRP) of dog pancreas with micrococcal nuclease results in the stepwise cleavage of the 300 nucleotide 7SL RNA moiety producing five major fragments approximately 220 (1), 150 (2), 72 (3), 62 (4) and 45 (5) nucleotides long. The RNA molecule is initially cut once yielding fragments 1 and 3. Further degradation releases fragments 2, 4 and 5. The introduction of the first nick into the 7SL RNA does not alter the structure nor the function of the SRP. Further degradation of the RNA results in disruption and loss of activity of the particle. The sequence of the RNA fragments shows that the nuclease causes discrete cuts in the RNA with minimal nibbling indicating that only few sites are accessible to the action of the enzyme. The five major products of nuclease digestion together span almost the entire length of the 7SL RNA. Nicking occurs mainly around the boundary region between the central S sequence and the flanking Alu sequences constituting the 7SL RNA (1). The S fragment is bound to the four largest polypeptides while the 5' and 3' Alu fragments are associated with the two smallest protein constituents of the SRP.

The signal-recognition particle (SRP) is a ribonucleoprotein (RNP) complex consisting of six different polypeptide chains and a 7SL RNA. It participates in initiating the translocation of proteins across the membrane of the endoplasmic reticulum, SRP was disassembled in 2 M KCl into three components, one RNP composed of 7SL RNA and the 54-kDa and 19-kDa proteins, and two heterodimers consisting of the 72/68-kDa and the 14/9-kDa proteins respectively. The 54-kDa protein could be released from the RNP subparticle by chromatography on DEAE-Sepharose in Mg2 ' -depleted buffer, while the 19-kDa protein remained bound to the 7SL RNA.The domain structure of SRP proteins was probed by using mild elastase treatment and protein-specific antibodies. It was found that the 72, 68, 54 and 19-kDa SRP proteins were proteolytically processed in distinct steps. Most remarkably a protein fragment of 55-kDa, generated from the 72-kDa SRP protein, and a 35-kDa fragment from the 54-kDa SRP protein were both released from the RNP particle. Fragments generated from the 68-kDa protein and detectable with the anti-(68-kDa protein) antibody remained associated with the RNP particle. Cleavage of the SRP proteins by elastase at 2.5 pg/ml resulted in partial loss of activity, while 10 lg/ml caused complete inactivation of the particle. Neither the elongation arrest of IgG light chain nor its translocation across SRP-depleted microsomal membranes was promoted. The implications of these results on the possible interaction between the SRP subunits are discussed.Secretory and membrane proteins are translocated across or inserted into the membrane of the endoplasmic reticulum (ER) [l]. This process is mediated by the signal-recognition particle (SRP) and docking protein, the receptor for SRP in the endoplasmic reticulum membrane [2 -51.Details of the functions of SRP and docking protein have been elucidated by using the wheat germ cell-free system and microsomal membranes derived from dog pancreas [4, 6, 71. It has been found that SRP interacts with the signal sequence in nascent secretory proteins and can arrest polypeptide chain elongation after about 70 and more amino acids have been polymerized [4,7, 81. When the arrested complex binds to the docking protein in the endoplasmic reticulum membrane, translation resumes and translocation of the nascent polypeptide chain across the membrane is initiated. According to these results, SRP would have at least three functional interactions: (a) recognition of nascent secretory polypeptides, (b) arresting elongation, and (c) interacting with the docking protein. This complex set of functions is probably reflected in the structural complexity of SRP.SRP is a rod-shaped RNP particle composed of six polypeptides with molecular masses of 72,68,54,19,14 and 9 kDa and one 7SL RNA molecule of 300 bases [9-111. The 7SL RNA can be divided into three distinct segments, the central 'S fragment', which contains a unique sequence of about 155 nucleotides, and the two flanking regions which show highCorrespondence ...

The secondary structure of zein mRNA affects its translational potential. Here we show that in a cell‐free system the translation efficiency of zein mRNA containing inverted repeats in the 5′‐ and 3′‐untranslated regions is reduced. This translational block is released after deletion of the 3′‐inverted repeat. We conclude that the translational block is caused by hybrid formation between the two inverted repeats. The translational efficiency of zein mRNAs, is also affected by varying the length or the primary structure of the 5′‐untranslated region.