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Membrane cofactor protein (CD46) is a basolateral protein that is not endocytosed. Importance of the tetrapeptide
A Maisner1, G Zimmer, M K Liszewski
1Institut für Virologie, Philipps-Universität Marburg, D-35037 Marburg, Germany.
Abstract:
Membrane cofactor protein (MCP) is a widely distributed complement regulatory protein that is expressed on the basolateral surface of polarized epithelial cells. The basolateral targeting of the BC1 isoform of MCP was analyzed by generating deletion mutants and point mutants within the cytoplasmic tail of 16 amino acids. A sequence of four amino acids, FTSL, was found to be indispensable for the basolateral transport of MCP. This tetrapeptide has two unique features compared with the targeting motifs of other basolateral proteins: (i) it contains a phenylalanine rather than a tyrosine at position 1; (ii) it is located at the very COOH-terminal end. Replacement of the phenylalanine or the leucine by an alanine resulted in a nonpolarized delivery to the cell surface. On the other hand, substitution of a tyrosine for the phenylalanine did not affect the basolateral transport of MCP. The latter mutant, however, was efficiently internalized, whereas the wild type protein was not subject to endocytosis. Our results indicate that the targeting signal YXX-large aliphatic that is involved in various sorting events has been modulated in MCP in such a way that it allows basolateral transport but not endocytosis.
Insights
Membrane cofactor protein (MCP) uses a unique FTSL sequence in its tail for basolateral transport in epithelial cells. This signal directs polarized delivery while preventing endocytosis.
Area of Science:
- Cell biology
- Molecular biology
- Protein trafficking
Background:
- Membrane cofactor protein (MCP) regulates complement.
- MCP is found on the basolateral surface of polarized epithelial cells.
Purpose of the Study:
- To analyze the basolateral targeting of the BC1 isoform of MCP.
- To identify the specific amino acid sequence responsible for MCP's polarized transport.
Main Methods:
- Generation of deletion and point mutants in the MCP cytoplasmic tail.
- Analysis of protein localization and cell surface delivery using mutant constructs.
Main Results:
- A tetrapeptide sequence, FTSL, is essential for basolateral transport of MCP.
- Mutations in FTSL resulted in nonpolarized cell surface delivery.
- A phenylalanine-to-tyrosine substitution allowed basolateral transport but increased endocytosis.
Conclusions:
- The FTSL motif is a novel basolateral targeting signal.
- MCP's targeting signal is modulated for polarized delivery and resistance to endocytosis, unlike typical YXX-large aliphatic motifs.