Heterologous Expression of Photoactivated Adenylyl Cyclase (PAC) Genes from the Flagellate Euglena gracilis in Insect Cells

The unicellular, green flagellate wild-type Euglena gracilis (strain Z) possesses two genes of the photoactivated adenylyl cyclase (PAC) family. The corresponding gene products were found to be responsible for step-up (but not step-down) photophobic responses as well as both positive and negative phototaxis. The proteins consist of two PACα (Mr 105 kDa) and two PACβ (90 kDa) subunits. In an effort to produce sufficient amounts of PAC proteins, several routes of overexpression have been tried including homologous expression in Euglena and heterologous expression in Escherichia coli. All these approaches were hampered by low yield or formation of inclusion bodies. Therefore we decided to attempt a heterologous expression in an insect cell line. PACα and PACβ were separately cloned in the transfer vector pBacPAK9 with a His tag attached. The transfer vector was subsequently cotransfected via baculovirus into the insect cells and amplified. For the expression both recombinant viruses (containing PACα and PACβ, respectively) were cotransfected simultaneously into insect cells. The expressed proteins were analyzed in Western blots using PACα and PACβ antibodies. Most of the proteins were found to be in soluble form in high yield. The recombinant PAC proteins were purified via their attached His tag on an anti-His resin. Adenylyl cyclase activity was quantified after blue-light excitation using a cAMP enzyme immunoassay kit.