That is plenty of time. Interline checkin is the norm. Your bag will be checked through to LAX. You will pick up your bag in SEA go through customs and then drop it off again before continuing on to the AS gates.

A tip: don't follow the signs from customs to the AS concourse. You will exit security and have to come back through. Instead, go upstairs to the NW concourse gates and find the gate for the shuttle bus to the AS concourse.