Membrane cofactor protein (CD46) of complement. Processing differences related to alternatively spliced cytoplasmic...

Area of Science:

• Immunology
• Cell Biology
• Virology

Background:

• Membrane Cofactor Protein (MCP, CD46) is a regulatory protein involved in complement inhibition and measles virus entry.
• MCP exists in four isoforms, differing in their cytoplasmic tails (CYT-1 and CYT-2).
• Previous observations indicated variable intracellular processing of MCP precursors.

Purpose of the Study:

• To characterize the intracellular transport and processing of individual MCP isoforms.
• To determine the role of cytoplasmic tails in MCP intracellular trafficking.
• To identify specific sequences within the MCP tail that regulate endoplasmic reticulum exit.

Main Methods:

• Pulse-chase analyses were performed on cells expressing different MCP isoforms and mutants.
• Chimera constructs were created to assess the influence of tail orientation on processing.
• Site-directed mutagenesis, including deletions within the CYT-1 tail, was employed.

Main Results:

• MCP isoforms with CYT-1 tails exhibited faster maturation (t1/2: 10-13 min) compared to CYT-2 tails (t1/2: 35-40 min).
• Tail-less MCP precursors showed significantly reduced processing rates (t1/2: 160-165 min).
• Deletion of the terminal FTSL sequence in CYT-1 significantly slowed processing (t1/2: 30-32 min), indicating its role in ER exit.

Conclusions:

• The carboxyl-terminal tail is the primary determinant of MCP intracellular processing rate.
• The FTSL tetrapeptide at the C-terminus of CYT-1 facilitates efficient exit from the endoplasmic reticulum.
• Understanding MCP trafficking is crucial for its regulatory functions and as a viral receptor.