Gram-negative bacteria commonly interact with eukaryotic host cells by using type III secretion systems (TTSSs or secretons). TTSSs serve to transfer bacterial proteins into host cells. Two translocators, IpaB and IpaC, are first inserted with the aid of IpaD by Shigella into the host cell membrane. Then at least two supplementary effectors of cell invasion, IpaA and IpgD, are transferred into the host cytoplasm. How TTSSs are induced to secrete is unknown, but their activation appears to require direct contact of the external distal tip of the apparatus with the host cell. The extracellular domain of the TTSS is a hollow needle protruding 60 nm beyond the bacterial surface. The monomeric unit of the Shigella flexneri needle, MxiH, forms a superhelical assembly. To probe the role of the needle in the activation of the TTSS for secretion, we examined the structurefunction relationship of MxiH by mutagenesis. Most point mutations led to normal needle assembly, but some led to polymerization or possible length control defects. In other mutants, secretion was constitutively turned "on." In a further set, it was "constitutively on" but experimentally "uninducible." Finally, upon induction of secretion, some mutants released only the translocators and not the effectors. Most types of mutants were defective in interactions with host cells. Together, these data indicate that the needle directly controls the activity of the TTSS and suggest that it may be used to "sense" host cells.Shigella flexneri causes bacillary dysentery in humans, a disease characterized by invasion of, massive inflammation in, and destruction of the colonic mucosa. The genes required for S. flexneri invasion are clustered on a 31-kb fragment of a large virulence plasmid (1). Within this region, the mxi/spa operons encode a type III secretion system (TTSS 5 or secreton), and the ipa/ipg operons encode five effector proteins abundantly secreted early in invasion, IpaA to -D and IpgD.Type III secretons are essential determinants of the interaction of many Gram-negative bacteria with animal or plant hosts, and they translocate bacterial proteins into eukaryotic host cells to manipulate them during infection. TTS apparatuses are encoded by ϳ25 genes (2), nearly all being essential for function. These devices perform regulated post-translational and co-translational protein translocation across three biological membranes (two from the bacterium and one from the host cell). In Shigella, IpaB to -D are essential for invasion and give rise to the TTSS translocon (3, 4) through which at least two other proteins, IpaA and IpgD, are thought to be translocated. Sequence similarities exist between components of TTSSs and those of the prokaryotic flagellar assembly machinery (5). TTSSs and flagella share all but host cell contact-mediated TTSS induction and the ability to translocate proteins into eukaryotic cells. 10 TTSS proteins are similar in sequence and/or membrane topology to the cytoplasmic or inner membrane proteins of flagellar hook-basal bodies (6, 7)...

Plasma alpha2HS-glycoprotein is specifically accumulated in calcified tissues. In the present studies this glycoprotein was isolated from plasma and after iodination with iodine-125 was injected intravenously into young rabbits. The tissue distribution and plasma disappearance rate of this radioactively labeled material were determined. Of the various tissues studied, bone showed the greatest retention of labeled glycoprotein expressed as percentage of the injected dose per gram tissue relative to the plasma content. The rate of loss of iodinated alpha2HS-glycoprotein from plasma was similar to that of alpha2HS-glycoprotein labeled endogenously by using 14C-glucosamine or 3H-glucosamine. The uptake of exogenously labeled 125I-alpha2HS-glycoprotein into bone tissue expressed as a percentage of the injected dose was similar to that of endogenously labeled 14C-alpha2HS-glycoprotein. These results suggest that the 125I-labeled material can be used to study further the metabolism of alpha2HS-glycoprotein by bone tissue.

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