MLL is proteolytically processed in vivo.

(A) Expression of MLL proteins in various cell lines. Cell lysates were analyzed by immunoblot with anti-MLL N-terminal (rpN1, mmN3) or C-terminal antibodies (mmC2, mmC3) or antibodies specific for p300 (RW128) or MOZ (N13-2), as denoted beneath the panels. Identities of cell lines are indicated above the gel lanes. The band corresponding to predicted 430-kDa full-length MLL is indicated by a plus mark. Bands corresponding to MLL N- and C-terminal fragments, p300 and MOZ, are indicated by asterisks. Migration of molecular weight standards is shown on the left. (B) BOSC23 cells were transfected with empty pLNCX vector (vector) or pLNCX expressing full-length MLL with an HA tag at its C-terminus [MLL(H)]. Cell lysates were analyzed by immunoblot with anti-MLL N-terminal antibodies (rpN1, mmN3, mmN4, mmN6), anti-HA antibody 3F10, or anti-MLL C-terminal antibodies (mmC2, mmC3). (C) Expression of MLL and MLL-AF6 in HL-60 and ML-1 cells. Cell lysates were analyzed by immunoblot with anti-MLL N-terminal antibody (rpN1). MLL N-terminal fragment and MLL-AF6 are indicated by arrows. (D) Expression of MLL and MLL-p300 in transiently transfected BOSC23 cells. Following transfection with the constructs indicated above the gel lanes, cell lysates were analyzed by immunoblot with anti-MLL N-terminal antibody (mmN3). MLL N-terminal fragment and MLL-p300 are indicated by arrows.