Yes, SIGNATURE is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, G 2, N 1, A 1, T 1, U 1, R 1, E 1.

Is it a valid word?

SIGNATURE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

your name written in your own handwriting
a distinguishing style
a melody used to identify a performer or a dance band or radio/tv program
the sharps or flats that follow the clef and indicate the key
a sheet with several pages printed on it; it folds to page size and is bound with other signatures to form a book

Also: signature tune, theme song, key signature.