FIGURE 1-24 Proposed model for the insertion of the /^-adrenergic receptor (AR) in the cell membrane. The model is based on hydropathicity analysis of the human /32AR. The standard one-letter codes for amino acid residues are used (See Appendix E). Hydrophobic domains are represented as transmembrane helices. Noted are the potential sites of N-linked glycosylation. Redrawn from Kobilka, B. K. et al. (1987). Proc. Natl. Acad. Sci. USA 84, 46.

spanning component, and the intracellular component. Each domain has its own unique biochemical properties reflecting its location and function. Frequently the membrane receptor comprises a single polypeptide chain where the N-terminus lies outside the cell and the C-terminus lies inside the cell, but some receptors may comprise subunits, i.e., ion channels.

The N-terminal portion of the membrane receptor usually possesses the ligand-binding domain, which will be defined by the primary amino acid composition of the peptide as well as by the resulting protein secondary structure. Sometimes the ligand-binding domain is determined by several membrane-spanning domains. Frequently, on the extracellular domain, there are glycosylation sites involving Asn-X-Ser/Thr for the attachment of N-linked oligosaccharides. The function of the oligosaccharide moieties is not known, but they could play a role in defining the ligand-

Tyrosine specific protein kinases

EGF receptor

PDGF receptor

Intracellular side

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