SRP Overview

The SRP cycle

The signal recognition particle (SRP) [gold] associates with ribosomes [gray] that are in the process of translating the mRNA [blue] for a secretory protein. The protein [red] has a signal at the N-terminus. Subsequently, the ribosome-bound SRP interacts with the SRP-receptor [green], a component of the ER membrane. Finally, SRP recycles to associate with another ribosome, and translation continues with the secretory protein transversing the membrane through a channel called the translocon. The molecular details of this essential biological process (see review, 1995, by H. Lütcke) have been unknown for many years, but significant progress has been made (2003 review by K. Nagai et al).

Components of the SRP

Shown is a crude map of the components of the eukaryotic SRP. The RNA helices [light blue] are numbered according to the nomenclature of Larsen & Zwieb and define the two functional domains of the SRP: the small domain (left; helices 2, 3, 4, and a portion of helix 5), and the large domain (right; a portion of helix 5, and helices 6, 7, and 8). (Helix 1 extends the 5'- and 3'-ends of the RNA and is present only in the SRPs of the archaea and certain bacteria.) There are six proteins [redish-green] in the mammalian SRP, named according to their approximate molecular weight in kDa. The translation-regulatory function resides in the small domain, while protein translocation and signal recognition are mediated through the large domain. Protein SRP54 recognizes the signal peptide (red) of the secretory protein. Overall, SRP is shaped like a dumbbell, about 240Å long. The SRP database has more information about the SRP components.)

Other SRP function cartoons:

SRP assembly:

Review:

EMBO J, 22, 3479-3485. (download/view pdf reprint).