How does the mutated protein differ from the original protein?

A mutant protein is the protein product encoded by a gene with mutation. Mutated protein can have single amino acid change (minor, but still in many cases significant change leading to disease) or wide-range amino acid changes by e.g. truncation of C-terminus after introducing premature stop codon.

What is protein? ... Most animal sources of protein, such as meat, poultry, fish, eggs, and dairy, deliver all the amino acids your body needs, while plant-based protein sources such as grains, beans, vegetables, and nuts often lack one or more of the essential amino acids.