NEOSPORA-CANINUM - TACHYZOITES EXPRESS A POTENT TYPE-I NUCLEOSIDE TRIPHOSPHATE HYDROLASE, BUT LACK NUCLEOSIDE DIPHOSPHATE HYDROLASE ACTIVITY

We have identified type I nucleoside triphosphate hydrolase (NTPase; E C 3.6.1.3) activity, previously thought to be restricted to the virule nt strains of Toxoplasma gondii, in the cell extracts of Neospora cani num tachyzoites. Sequence analysis of a complete cDNA from Nc-l strain indicated that N. caninum NTPases shared approximately 69% identity t o the NTPases of T. gondii and are most similar to the NTPase-I isozym e. Southern blot analysis of genomic DNA and sequence analysis of two independent NTP clones from the Nc-l strain revealed the presence of m ultiple genes, at least two of which are transcribed. Substrate specif icity and K-m values for MgATP(2-) and MgADP(-) hydrolysis for recombi nant or partially purified native NcNTPase were the same as those for the type I isozyme (NTPase-I). Significantly, no type II enzyme (NTPas e-II) activity for NDP hydrolysis was detected in cell extracts of N. caninum, although it is universally present in all T gondii strains th at have been tested. This intriguing difference between these two clos ely related apicomplexan parasites may provide insight into the functi on of the NTPases during intracellular parasitism. (C) 1998 Academic P ress.