Following up a whopper deal with Biogen Idec Inc. about a year ago, Proteostasis Therapeutics Inc. (PTI) nailed down a pact with Astellas Pharma Inc. that brings up-front cash, a securities investment and potentially $400 million in milestone payments, ultimately gaining as much as $1.2 billion if Astellas exercises options for two more projects under the same terms and they succeed fully.