The SRP cycle

1.An emerging ER signal peptide 2.is captured by SRP (recognition step) 3.which directs the SRP/SP complex (targeting step) to the α subunit of SRP receptors in the ER membrane 4.allowing recruitment of a closed translocon 5.which opens to allow entry of the SP as a loop whilst the SRP/SRP receptor complex is dismantled and SRP is recycled. 6.Signal peptidase removes the SP, which is released into the ER membrane. There are millions of active translocons in the ER membrane, so at any one time there could be millions of cleaved signal peptides – which could disrupt the ER membrane.