TABLE 2.

Structural features of the OM components of protein secretion systems

Secretion system	OM component	Structural features			References
		Size of precursor protein (aa)a	Structural organization	Size of export channel (Å)
Sec dependent
AT	NalP	1,083	Monomer, 12-stranded β-barrel with N-terminal α-helix	10 by 12.5	52
TPS	HMW1B	545	Homotetramer, predicted 22-stranded β-barrel	27	67
CU	PapC	836	Dimer, β-barrel pore structure	20-30	29, 41
T2S	PulD	660	Homomultimer (12 monomers), extensive β-barrel structure predicted	76	49
ENP	CsgG?	277	?	?	43, 64
Sec independent
T1S	TolC	495	Homotrimer, each monomer contributing 4 β-strands to form 12-stranded β-barrel	35	38
T3S	YscC	607	Multimer, ring-like structure	40	9, 40
T4S	VirB9	293	Heterodimer, complexes with VirB7 via disulfide bonds	?	2

^aaa, amino acids.