Identification and characterization of two Ca2+/CaM-dependent protein kinases required for normal nuclear division in Aspergillus nidulans

Abstract

We utilized an expression screen to identify two novel Ca2+/calmodulin (CaM)-regulated protein kinases in Aspergillus nidulans. The two kinases, CMKB and CMKC, possess high sequence identity with mammalian CaM kinases (CaMKs) I/IV and CaMKKα/β, respectively. In vitro CMKC phosphorylates and increases the activity of CMKB, indicating they are biochemical homologues of CaMKKα/β and CaMKI/IV. The disruption of CMKB is lethal; however, when protein expression is postponed, the spores germinate with delayed kinetics. The observed lag corresponds to a delay in the G1-phase activation of the cyclin-dependent kinase NIMXcdc2. Disruption of cmkC is not lethal, but spores lacking CMKC also germinate with delayed kinetics and a lag in the activation of NIMXcdc2. Analysis of ΔcmkC suggests a role for CMKC in regulating the first and subsequent nuclear division cycles. We conclude that both CMKB and CMKC are required for the proper temporal activation of NIMXcdc2 as spores enter the cell cycle from quiescence and suggest that this relationship exists during the G1/S transition of subsequent cell divisions.