If you know the characteristics of protein like size, isoelectric pH. You can take cell extract run a 2 D gel and can cut the band corresponding to those characteristics.

This is the simplest way of extraction any body can think of.

For the seed's processing and the protocol you can follow this article

J Agric Food Chem. 2005 May 18;53(10):4245-52

Seeds contains various proteins...It depends on which kind/type of protein you want to isolate.