Is every protein encoded by just one gene?
There are proteins encoded by more than one gene. 

It would be a heterodimer protein of quaternary structure. One famous example would be haemaglobin, which is assembled from alpha and beta sub-units.

Also, one gene is capable of coding for multiple proteins!

https://en.wikipedia.org/wiki/Protein_dimer

https://en.wikipedia.org/wiki/Protein_quaternary_structure

https://en.wikipedia.org/wiki/Hemoglobin