Small peptide recognition sequence for intracellular sorting

Abstract

Increasing evidence indicate that complex arrays of short signals and recognition peptide sequence ensure accurate trafficking and distribution of transmembrane receptors and/or proteins and their ligands into intracellular compartments. Internalization and subsequent trafficking of cell-surface receptors into the cell interior is mediated by specific short-sequence peptide signals within the cytoplasmic domains of these receptor proteins. The short signals usually consist of small linear amino acid sequences, which are recognized by adaptor coat proteins along the endocytic and sorting pathways. In recent years, much has been learned about the function and mechanisms of endocytic pathways responsible for the trafficking and molecular sorting of membrane receptors and their ligands into intracellular compartments, however, the significance and scope of the short-sequence motifs in these cellular events is not well understood. Here a particular emphasis has been given to the functions of short-sequence signal motifs responsible for the itinerary and destination of membrane receptors and proteins moving into subcellular compartments.

Figure 1. Intracellular pathways of receptor-medicated endocytosis and trafficking

The binding of ligand to specific cell surface receptors leads to a selective recruitment of ligand-receptor complexes into clathrin-coated pits on the plasma membrane. The coated pit invaginates and pinches-off into a vesicle in the cytosol, which triggers the recruitment of adapter proteins and other interacting molecules. The clathrin -dependent routes require dynamin to achieve the fission of the membrane invaginations and vesicle internalization. The ligand-receptor complexes within the cargo entering via the clathrin pathway are usually directed to early endosomes. From the endosomes, the receptors and ligands are sorted to various subcellular locations, where the internalized molecules are either sorted to degradative compartments such as the late endosomes, and/or lysosomes, or recycled to the plasma membrane via recycling endosomes. The recycled molecules can participate in several rounds of endocytosis. Alternatively, the internalized cargo molecules may be sequestered in endosomes for a longer period of time and continue to spark signaling events. Some early and late endosomes also contain membrane structures in the lumen, which are referred to as multi-vesicular bodies (MVBs). The endosomal and lysosomal system can also transmit and receive cargo from the trans-Golgi network (TGN). The critical molecules involved in the trafficking at different locations have been indicated as AP-1, AP-2, AP-3, AP-4, Dab-1, GGA, PACS-1, and TIP.

Figure 2. Role of the GDAY motif in the internalization and trafficking of wild-type mutant guanylyl cyclase/natriuretic peptide receptor-A

HEK-293 cells expressing wild-type, G920A, D921A, Y923A or GDAY/AAAA mutant receptors were preincubated for 1 h at 4 °C with labeled ¹²⁵I-atrial natriuretic peptide (¹²⁵I-ANP). Surface ¹²⁵I-ANP binding to wild-type and mutant receptors was performed at 4°C. Subsequently, cells were warmed at 37 °C for the indicated time periods to permit the internalization of ligand–receptor complexes into the cell interior. Internalization of receptor was quantified at different time points. Copyright @2005 The Biochemical Journal.