Following activation of a receptor tyrosine kinase such as the EGF receptor, adaptor proteins assemble at phosphotyrosine sites. In the sequence Grb2, SOS, Ras, Raf, MEK, ERK, the function of SOS is to act as a:

• A Guanine nucleotide exchange factor that loads Ras with GTP ✓
• B Tyrosine kinase that phosphorylates Raf
• C GTPase activating protein that terminates Ras signalling
• D Transcription factor that translocates to the nucleus after ERK phosphorylation

Explanation

SOS (son of sevenless) is recruited to the membrane by the SH2-containing adaptor Grb2 and functions as a guanine nucleotide exchange factor. It displaces GDP from Ras so GTP can bind, converting Ras to its active conformation. Active Ras then recruits Raf, beginning the MAP kinase cascade. GAPs perform the opposite reaction and inactivate Ras, ruling out C, and Ras is a small GTPase, not a kinase target of SOS, ruling out B.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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