Today, we are able to determine the sequence of proteins, and to your surprise, there are lots of proteins that do not start with methionine (despite the fact that methionine is the start codon needed to initiate translation). Why do you think this is?

a) the protein is post transcriptionally modified and cleaved

b) the protein starts translating at a different start sequence than the original one that is coding for methionine. 

c) the protein has a special aminoacyl tRNA that binds to another amino acid aside from methionine at the start sequence in the mRNA. 

d) all proteins start with  methionine. Protein sequencing is clearly not accurate.

e) None of the above. 

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The correct answer is A. 

The answer has to do with understanding the mechanism of protein translation and modification. In particular, proteins that are generally supposed to be either in the member or outside the cell are translated into the ER and from there, they actually get post-translationally modified in the golgi before they are fully described as “mature” and proper. In this modification scheme, what you can get is splicing of different regions of the protein, and if the region happens to be the beginning of the protein, then what you’ll get is a protein that does not end up with a methionine at the N terminus. This is described by option A, and that is why A is the correct answer.