In the sense that they contribute to umami taste, yes. But most commonly the nucleotides inosinate (from meat and fish) and guanylate (from dried mushrooms) are the other molecules that provide umami flavors.
I'm pretty sure the MSG disassociates into the sodium ion and the glutamate, since it's soluble in water. I only use the term MSG to refer to the additive when it's in the salt form in a dry jar.
Side note: it's really funny if you think about it, umami is basically just the taste of amino acids and nucleic acids, which presumably makes sense since the body uses them so much (beyond just making protein and DNA/RNA).
