Vesicles carry enzymes to modify and sort proteins. The enzyme composition of each individual sac is constantly changing as it ages and passes from cis to trans face. Recent evidence favors this model. See Science article for an extended discussion. Most organelle proteins must be imported .
Proteins can pass through both mito.
See Becker fig. 4.10 B. Pore Function 1. Active transporter for big molecules . See Becker fig. 16-30 for a model.