How are different coat proteins recruited to different sites within the cell?

There are three known classes of coat proteins: clathrin,COPI,and COPII.Small GTPases recruit each to membranes.Sar1 recruits COPII.Sar1 itself is activated by a guanine nucleotide exchange factor,Sec12,an ER integral membrane protein.ARF recruits both clathrin and COPII.How ARF,or different isoforms of ARF,and guanine nucleotide exchange factor(s)recruit different coat protein/adapters to different sites is not yet known.