This is a good question and one that could be asked for TTG start
codons as well. The thinking has been that f-met is at the amino
terminal end of these proteins but there is little or data to back
this up (if someone is aware of such data I'd appreciate it if this
was brought to my attention).
Although I have worked with two TTG start codon proteins (the PC-1
beta-lactamase from S. aureus and the endopeptidase lysostaphin both
are secreted proteins and the amino terminal ends are cleaved off so
no determination has been made as to whether or not the first amino
acid residue is actually an f-met.
One result (unpublished) I generated when trying to over-express
recombinant lysostaphin (in B. subtilis and B. sphaericus) was that
changing the start codon from TTG to ATG had no affect on the level
of protein production. This implies that if there is an f-met
amino-acyl tRNA which recognizes TTG it is in adequate abundance
within the cell. (Alternatively there may be some other factor that
is more limiting that f-met amino-acyl tRNA levels.)
Another observation is that many of the proteins which carry these
(GTG and TTG) start codons are extra-cytoplasmic. Does this
represent some part of the mechanism for protein secretion/transport
out of the cell??
Steve Projan
Wyeth-Ayerst Research
