instruction:
In this task, you are given an abstract of article. Your task is to generate title for this article. Preferred titles are under thirty words.
question:
Virulence in Staphylococcus aureus is largely under control of the accessory gene regulator (agr) quorum-sensing system. The AgrC receptor histidine kinase detects its autoinducing peptide (AIP) ligand and generates an intracellular signal resulting in secretion of virulence factors. Although agr is a well-studied quorum-sensing system, little is known about the mechanism of AgrC activation. By co-immunoprecipitation analysis and intermolecular complementation of receptor mutants, we showed that AgrC forms ligand-independent dimers that undergo trans-autophosphorylation upon interaction with AIP. Remarkably, addition of specific AIPs to AgrC mutant dimers with only one functional sensor domain caused symmetric activation of either kinase domain despite the sensor asymmetry. Furthermore, mutant dimers involving one constitutive protomer demonstrated ligand-independent activity, irrespective of which protomer was kinase deficient. These results demonstrate that signalling through either individual AgrC protomer causes symmetric activation of both kinase domains. We suggest that such signalling across the dimer interface may be an important mechanism for dimeric quorum-sensing receptors to rapidly elicit a response upon signal detection.
answer:
Symmetric signalling within asymmetric dimers of the Staphylococcus aureus receptor histidine kinase AgrC.

question:
The lower respiratory tract of cigarette smokers contains an increased amount of iron that is predominantly sequestered within alveolar macrophages (AM), but is also present in alveolar epithelial fluid. Extracellular ferritin-bound iron could potentially be released by reductants present in cigarette smoke and catalyze generation of highly reactive hydroxyl radicals capable of causing oxidant injury. To determine whether AM are a source of alveolar extracellular ferritin and iron, we assessed in vitro release of iron, ferritin, and transferrin by AM recovered by bronchoalveolar lavage (BAL) of 27 healthy subjects including nine nonsmokers (NS), nine light smokers (LS), and nine heavy smokers (HS). Release of iron in vitro over 20 h was increased in AM recovered from LS (2.24 +/- 0.21 nmol/10(6) AM/20 h, p < 0.001) and HS (3.11 +/- 0.32 nmol/10(6) AM, p < 0.001) compared with NS (1.28 +/- 0.08 nmol/10(6) AM). Release of ferritin in vitro over 20 h was also increased in AM recovered from LS (71 +/- 24 ng/10(6) AM, p < 0.05) and HS (176 +/- 35 ng/10(6), p < 0.001) compared with NS (18 +/- 3 ng/10(6) AM). AM recovered from 12 smokers (8 HS, 4 LS) contained greater than 10 nmol of iron per 10(6) cells. These iron-loaded AM released a greater percentage of cell ferritin stores in vitro over 4 h (8.4% +/- 1.1, p < 0.01) than did AM from NS (3.2% +/- 0.6). Release of lactate dehydrogenase (LDH) over 4 h was substantially less (2.9% +/- 0.3, p < 0.001) than ferritin release.(ABSTRACT TRUNCATED AT 250 WORDS)
answer:
Increased release of ferritin and iron by iron-loaded alveolar macrophages in cigarette smokers.

question:
BACKGROUND Microtubules are built from linear polymers of - tubulin dimers (protofilaments) that form a tubular quinary structure. Microtubules assembled from purified tubulin in vitro contain between 10 and 16 protofilaments; however, such structural polymorphisms are not found in cells. This discrepancy implies that factors other than tubulin constrain microtubule protofilament number, but the nature of these constraints is unknown.
RESULTS Here, we show that acetylation of MEC-12 -tubulin constrains protofilament number in C. elegans touch receptor neurons (TRNs). Whereas the sensory dendrite of wild-type TRNs is packed with a cross-linked bundle of long, 15-protofilament microtubules, mec-17;atat-2 mutants lacking -tubulin acetyltransferase activity have short microtubules, rampant lattice defects, and variable protofilament number both between and within microtubules. All-atom molecular dynamics simulations suggest a model in which acetylation of lysine 40 promotes the formation of interprotofilament salt bridges, stabilizing lateral interactions between protofilaments and constraining quinary structure to produce stable, structurally uniform microtubules in vivo.
CONCLUSIONS Acetylation of -tubulin is an essential constraint on protofilament number in vivo. We propose a structural model in which this posttranslational modification promotes the formation of lateral salt bridges that fine-tune the association between adjacent protofilaments and enable the formation of uniform microtubule populations in vivo.
answer:
Posttranslational Acetylation of -Tubulin Constrains Protofilament Number in Native Microtubules