Where is the user table gets updated?

I'm trying to understand how user registration form changes get written to the DB in order to understand what is the apropriate hook to use in my module.
So
i changed the signature field and pressed submit
i added a print in the _db_query in database.mysql.inc

but couldn't find there any reference to signature,
the database did get the update so it was happening somewhere else

i'm a bit puzzeled
anyone knows?

tnx,
hed