The inner membrane is surrounded by the outer nuclear membrane, which is continuous with the membrane of the ER. The outer nuclear membrane is studded with ribosomes engaged in protein synthesis.

TLS Online TPP Program

#Id: 7931

#Unit 2. Cellular Organization

Microsomes

when cells are homogenized, the rough ER breaks up into small closed vesicles with ribosomes on the outside

TLS Online TPP Program

#Id: 7932

#Unit 2. Cellular Organization

Signal sequence

ER targeting sequence, located at the N-terminus of the protein

TLS Online TPP Program

#Id: 7933

#Unit 2. Cellular Organization

Signal sequences

contain one or more positively charged amino acids adjacent to a continuous stretch of 6–12 hydrophobic residues (known as the hydrophobic core)

TLS Online TPP Program

#Id: 7934

#Unit 2. Cellular Organization

The two key components in protein targeting in ER:

Signal recognition particle (SRP)

SRP receptor

TLS Online TPP Program

#Id: 7935

#Unit 2. Cellular Organization

The SRP is a cytosolic ribonucleoprotein particle that transiently binds to both the ER signal sequence in a nascent protein and the large (60S) ribosomal subunit, forming a large complex.

TLS Online TPP Program

#Id: 7936

#Unit 2. Cellular Organization

The SRP is made up of six proteins bound to a 300-nucleotide RNA, which acts as a scaffold for the hexamer.