Calmodulin activation of an endoplasmic reticulum-located calcium pump involves an interaction with the N-terminal autoinhibitory domain.
To investigate how calmodulin regulates a unique subfamily of Ca(2+) pumps found in plants, we examined the kinetic properties of isoform ACA2 identified in Arabidopsis. A recombinant ACA2 was expressed in a yeast K616 mutant deficient in two endogenous Ca(2+) pumps. Orthovanadate-sensitive (45)Ca(2+) transport into vesicles isolated from transformants demonstrated that ACA2 is a Ca(2+) pump. Ca(2+) pumping by the full-length protein (ACA2-1) was 4- to 10-fold lower than that of the N-terminal truncated ACA2-2 (Delta2-80), indicating that the N-terminal domain normally acts to inhibit the pump. An inhibitory sequence (IC(50) = 4 microM) was localized to a region within valine-20 to leucine-44, because a peptide corresponding to this sequence lowered the V(max) and increased the K(m) for Ca(2+) of the constitutively active ACA2-2 to values comparable to the full-length pump. The peptide also blocked the activity (IC(50) = 7 microM) of a Ca(2+) pump (AtECA1) belonging to a second family of Ca(2+) pumps. This inhibitory sequence appears to overlap with a calmodulin-binding site in ACA2, previously mapped between aspartate-19 and arginine-36 (J.F. Harper, B. Hong, I. Hwang, H.Q. Guo, R. Stoddard, J.F. Huang, M.G. Palmgren, H. Sze ¿1998 J Biol Chem 273: 1099-1106). These results support a model in which the pump is kept "unactivated" by an intramolecular interaction between an autoinhibitory sequence located between residues 20 and 44 and a site in the Ca(2+) pump core that is highly conserved between different Ca(2+) pump families. Results further support a model in which activation occurs as a result of Ca(2+)-induced binding of calmodulin to a site overlapping or immediately adjacent to the autoinhibitory sequence.
Hwang
I
I
Department of Cell Biology and Molecular Genetics, and Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742, USA.
Harper
J F
JF
Liang
F
F
Sze
H
H
D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000254:growth & development / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism*
C066778:ACE2 protein, S cerevisiae; D002147:Calmodulin; D004268:DNA-Binding Proteins; D005656:Fungal Proteins; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D000252:Calcium-Transporting ATPases; D002118:Calcium