True and false alternative transcripts of calcium-dependent protein kinase CPK9 and CPK3a genes in Vitis amurensis

Abstract

Calcium-dependent protein kinases (CDPKs) are major Ca2+ sensors in plants playing important roles in plant development and stress responses. In the present study, RT-PCR analysis revealed that VaCPK3a and VaCPK9 cDNAs lacking extensive regions in the kinase, autoinhibitory, and Ca2+-binding domains were numerous in probes derived from wild grapevine Vitis amurensis. Most of the VaCPK3a and VaCPK9-modified transcripts lacked canonical splice sites and possessed short direct repeated sequences (SDRs) instead. Three VaCPK9 transcript variants were generated using canonical 5′GT and 3′AG splice sites and lacked several subdomains in the kinase domain, including ATP-binding site, which is known to be indispensable for kinase activities. These observations indicate that post-transcriptional mRNA processing might lead to production of CDPKs with abolished phosphorylation activities. Recombinant proteins VaCPK3aSF2, lacking autoinhibitory and Ca2+-binding domain, and VaCPK3aSF3, lacking VIII–XI kinase subdomains and autoinhibitory domain, phosphorylated exogenous substrate in a Ca2+-independent manner. However, reverse transcription at +65 °C using heat-stable reverse transcriptase (RT) markedly lowered abundance of the unusual VaCPK3a and VaCPK9 cDNAs with SDRs, while it did not eliminate VaCPK9 cDNAs generated using canonical splice sites. The results show that VaCPK9 gene undergoes unproductive alternative splicing mediated by canonical splice sites to generate three mRNA isoforms lacking important catalytic kinase subdomains. The unusual VaCPK3a and VaCPK9 transcripts with SDRs are likely to be false alternative transcripts generated by RT template switching in vitro. The data demonstrate that using non-thermostable RTs for studying alternative splicing are not appropriate.