GO:0035597:   N6-isopentenyladenosine methylthiotransferase activity
Catalysis of the methylthiolation (-SCH3 addition) at the C2 of the adenosine ring of N6-isopentenyladenosine (i6A) in tRNA, to form 2-methylthio-N6-isopentenyladenosine (ms2i6A).