What cargo do signal recognition particles typically transport?

Signal recognition particles (SRPs) primarily transport proteins that are destined for the endoplasmic reticulum (ER). This process is a crucial step in the synthesis and proper localization of proteins that will either be secreted from the cell, incorporated into the cell membrane, or sent to lysosomes.

The mechanism begins when a ribosome synthesizes a polypeptide chain that contains a signal sequence. This sequence is recognized by the SRP, which temporarily halts translation and guides the ribosome-polypeptide complex to the ER membrane. Upon reaching the ER, the SRP interacts with a receptor that facilitates the transfer of the ribosome to a translocon, allowing translation to resume and for the nascent protein to be translocated into the lumen of the ER.

This pathway is essential for proper protein sorting within the cell, ensuring that proteins that need to be secreted or integrated into membranes are correctly processed and routed. Consequently, proteins that are not designed to enter the ER are not involved in this particular trafficking mechanism, making it clear why options concerning RNA molecules, nuclear proteins, or lysosomal enzymes do not match the role of SRPs.