The E592K variant preserves association of SF3B1 with SUGP1. HEK293T cells were transfected with His6-FLAG-SF3B1 variants and subjected to affinity purification with anti-DYKDDDDK (FLAG) antibody. (A) Silver-stained protein gel, with arrow pointing to the size of SUGP1, which is decreased in K700E but not E592K eluate. (B) Western blot showing decreased SUGP1 in K700E, but not E592K, eluate. PHF5A is present with all SF3B1 variants. (C) Reprobing with anti-SF3B1 shows native and His6-FLAG–tagged protein levels. E, E592K; K, K700E; KD, kilodaltons; M, marker; Vec, vector only; W, wild type SF3B1.