A domain refers to a discrete, locally folded unit of tertiary structure. It is assumed that domains of common structure have common functions, but not necessarily vice versa. A motif refers to a region with a particular combination of secondary structural elements such as alpha helices, beta sheets, loops, turns, etc. Common structure implies common function. Important tool in analysis of results from human genome project.
Common structure implies common origin.
There are two ways to get a collection of similar proteins 1. Antibody can be membrane bound or secreted. Fate of antibody depends on whether peptide has a stop transfer sequence or not. If has stop transfer, locks into membrane of ER and goes thru Golgi etc. If has no stop transfer, enters lumen of ER, goes thru Golgi etc.