Complex sentence: Five of the schools (California, Stanford, USC, UCLA, and Washington) immediately formed the Athletic Association of Western Universities (AAWU, informally the Big Five), the predecessor to the modern Pac-12 Conference.
Split into simpler sentences: Five of the conference's members, California, Stanford, USC, UCLA, and Washington, immediately formed the Athletic Association of Western Universities (AAWU, informally the Big Five). The AAWU is the predecessor to the modern Pac-12 Conference.