A calmodulin-dependent translocation pathway for small secretory proteins.

Area of Science:

• Cell biology
• Molecular biology
• Protein translocation

Background:

• Metazoans secrete numerous small proteins crucial for cellular functions.
• Rapid synthesis of these proteins limits cotranslational translocation into the ER.
• The pathway for small secretory protein entry into the ER remains unclear.

Purpose of the Study:

• To elucidate the mechanism of ER entry for small secretory proteins.
• To identify the factors involved in the translocation of these proteins.
• To investigate the role of calmodulin in this process.

Main Methods:

• In vivo and in vitro translocation assays.
• Analysis of cytosolic intermediates using protein binding studies.
• Calmodulin inhibition experiments.

Main Results:

• Small secretory proteins enter the ER posttranslationally via a transient cytosolic intermediate.
• Calmodulin selectively binds to signal peptides within this intermediate.
• Calmodulin prevents precursor aggregation, degradation, and non-specific binding.
• Calmodulin inhibition disrupts small protein translocation.

Conclusions:

• A novel mammalian posttranslational pathway for small protein secretion is established.
• Calmodulin functions as a crucial chaperone for small secretory protein precursors.
• This finding reveals an unexpected role for calmodulin in protein translocation.