SOS2 (Son of sevenless homolog 2) is a 160-170kD protein that regulates Ras signaling. It is widely expressed, and serves as a mediator of guanine nucleotide phosphate exchange. In the resting cell, cytoplasmic SOS2 forms a heterodimer with Grb2, and a heterotrimer with Eps8 and E3b1. Upon RTK activation, the SOS heterodimer is recruited to the cell membrane, where it contacts GDP-bound Ras. This allows it to facilitate a GTP-for-GDP exchange that activates Ras. The heterotrimer interacts with Rac on actin filaments. SOS2 activity parallels that of SOS1. Relative to SOS1, however, SOS2 binds Grb2 with higher affinity and shows less biological activity due to a shorter half-life. Human SOS2 is 1332aa in length and contains one histone fold (aa97-169), PH (aa439-546) and REM (aa595-739) domains that interact with Ras, and a Pro-rich region that binds to Grb2 (aa1126-1242).