Catalyzes the ATP-dependent addition of one alpha-linked L-glutamate molecule to coenzyme gamma-F420-2, producing alpha-F420-3, the major form of coenzyme F420 found in M.jannaschii. Thus, caps the gamma-polyglutamate tail of coenzyme F420 with a terminal alpha-linked glutamate. Prefers ATP to other purine nucleotide triphosphates; GTP gives about 25% of the activity observed with ATP. Cannot catalyze the addition of the following amino acids or analogs: D-glutamate, beta-glutamate, L-aspartate, L-glutamine, L-alpha-aminoadipate, or D,L-2-amino-4-phosphono-butyrate. ATP + L-glutamate + oxidized coenzyme F420-2 = ADP + H(+) + oxidized coenzyme alpha-F420-3 + phosphate Binds 2 manganese ions per subunit. Inhibited by KCl. Thermostable. Cofactor biosynthesis; coenzyme F420 biosynthesis. Monomer. Belongs to the RimK family. CofF subfamily.