Disassembly and domain structure of the proteins in the proteins in the signal-recognition particle

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Abstract
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‐k Da protein) antibody remained associated with the RNP particle. Cleavage of the SRP proteins by elastase at 2.5 μg/ml resulted in partial loss of activity, while 10 μg/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.