Conserved amino acids at the C-terminus of rat phospholipase D1 are essential for enzymatic activity

Rat brain phospholipase D1 (rPLD1) has two highly conserved motifs [H(X)K(X)4D, denoted HKD] located at the N-terminal and C-terminal halves, which are required for activity. Association of the two halves is essential for rPLD1 activity, which probably brings the two HKD domains together to form a catalytic center. In the present study, we find that an intact C-terminus is also essential for the catalytic activity of rPLD1. Serial deletion of the last four amino acids, EVWT, which are conserved in all mammalian PLD isoforms, abolished the catalytic activity of rPLD1. This loss of catalytic activity was not due to a lack of association of the N-terminal and C-terminal halves. Mutations of the last three amino acids showed that substitutions with charged or less hydrophobic amino acids all reduced PLD activity. For example, mutations of Thr1036 and Val1034 to Asp or Lys caused marked inactivation, whereas mutation to other amino acids had less effect. Mutation of Trp1035 to Leu, Ala, His or Tyr caused complete inactivation, whereas mutation of Glu1033 to Ala enhanced activity. The size of the amino acids at the C-terminus also affected the catalytic activity of PLD, reduced activity being observed with conservative mutations within the EVWT sequence (such as T/S, V/L or W/F). The enzyme was also inactivated by the addition of Ala or Val to the C-terminus of this sequence. Interestingly, the inactive C-terminal mutants could be complemented by cotransfection with a wild-type C-terminal half to restore PLD activity in vivo. These data demonstrate that the integrity of the C-terminus of rPLD1 is essential for its catalytic activity. Important features are the hydrophobicity, charge and size of the four conserved C-terminal amino acids. It is proposed that these play important roles in maintaining a functional catalytic structure by interacting with a specific domain within rPLD1.