▁of ▁which ▁may ▁be ▁struct ur ally ▁related . ▁Since ▁the ▁first ▁evidence ▁of ▁a ▁mac rom ole cular ▁array ▁on ▁a ▁b acter ial ▁cell ▁wall ▁fragment ▁in ▁the ▁ 1 9 5 0 s ▁S - layer ▁structure ▁has ▁been ▁investig ated ▁extens ively ▁by ▁electron ▁micro sc opy ▁and ▁medium ▁resolution ▁images ▁of ▁S - layers ▁from ▁these ▁anal ys es ▁has ▁provided ▁useful ▁information ▁on ▁overall ▁S - layer ▁morph ology . ▁High - resol ution ▁structures ▁of ▁an ▁arch ae al ▁S - layer ▁protein ▁( MA 0 8 2 9 ▁from ▁M eth anos arc ina ▁ac et iv or ans ▁C 2 A ) ▁of ▁the ▁M eth anos arc inal es ▁S - layer ▁T ile ▁Prote in ▁family ▁and ▁a ▁b acter ial ▁S - layer ▁protein ▁( S bs B ), ▁from ▁Ge ob ac ill us ▁ste ar oth erm oph ilus ▁P V 7 2 , ▁have ▁recently ▁been ▁determined ▁by ▁X - ray ▁cry stal log raph y . ▁In ▁contrast ▁with ▁existing ▁cry stal ▁structures , ▁which ▁have ▁represented ▁individual ▁domains ▁of ▁S - layer ▁prote ins ▁or ▁minor ▁protein ace ous ▁components ▁of ▁the ▁S - layer , ▁the ▁MA 0 8 2 9 ▁and ▁S bs B ▁structures ▁have ▁allowed ▁high ▁resolution ▁models ▁of ▁the ▁M . ▁ac et iv or ans ▁and ▁G . ▁ste ar oth erm oph ilus ▁S - layers ▁to ▁be ▁proposed . ▁These ▁models ▁exhib it ▁hex ag onal ▁( p