The signal recognition particle receptor is a complex that contains two distinct polypeptide chains

Area of Science:

• Molecular Biology
• Cell Biology
• Protein Trafficking

Background:

• Signal recognition particle (SRP) and SRP receptor are essential for targeting secretory proteins to the endoplasmic reticulum (ER) membrane.
• The SRP receptor was previously thought to consist solely of a 69-kD alpha-subunit (SR alpha).

Purpose of the Study:

• To investigate the complete composition of the SRP receptor.
• To identify and characterize novel components of the SRP receptor complex involved in protein translocation.

Main Methods:

• Purification of SRP receptor using SRP-Sepharose affinity chromatography.
• Co-purification and immunological characterization of associated ER membrane proteins.
• Analysis of subunit complex stability (high salt) and co-fractionation (sucrose gradients).

Main Results:

• A 30-kD beta-subunit (SR beta) was identified as a novel, integral component of the SRP receptor.
• SR beta forms a stable complex with SR alpha, present in equimolar amounts in the ER membrane.
• Another 30-kD ER membrane protein (mp30) was found to co-purify but is distinct from SR beta and present in excess.

Conclusions:

• The SRP receptor is composed of both SR alpha and SR beta subunits, with SR beta being an integral component.
• The presence of SR beta was previously obscured by degradation products and co-purifying proteins like mp30.
• Mp30's interaction with SRP suggests a potential, yet uncharacterized, role in protein translocation.