Penicillin-binding protein 1A (PBP1A) is a bifunctional class A high-molecular-mass penicillin-binding protein that catalyzes both glycosyltransferase (GT) and D,D-transpeptidase (TP) activities essential for peptidoglycan biosynthesis. The N-terminal GT domain (EC 2.4.99.28) polymerizes glycan strands from lipid II precursors, while the C-terminal TP domain (EC 3.4.16.4) cross-links peptide stems to form the mature peptidoglycan sacculus. The enzyme is inner membrane-anchored with catalytic domains operating in the periplasmic space. In Gram-negative bacteria, PBP1A activity is regulated by outer membrane lipoprotein activators (LpoA/LpoP family). The TP domain is the target of beta-lactam antibiotics while the GT domain is inhibited by moenomycin. PBP1A functions redundantly with PBP1B (mrcB) for core peptidoglycan synthesis but shows stress-specific requirements.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030288 outer membrane-bounded periplasmic space | IEA GO_REF:0000118 | ACCEPT | Summary: PBP1A is an inner membrane-anchored protein with large periplasmic catalytic domains that operate at the peptidoglycan layer interface. The TreeGrafter annotation correctly places the functional site in the periplasmic space where the enzyme synthesizes and crosslinks peptidoglycan [Pazos & Vollmer 2021, Typas et al. 2010]. Reason: The localization is well-supported by the protein architecture: PBP1A has an N-terminal membrane anchor with the GT and TP catalytic domains extending into the periplasm where they act on lipid II and the nascent peptidoglycan layer. Supporting Evidence: file:PSEPK/mrcA/mrcA-deep-research-falcon.md aPBPs are inner-membrane anchored, with periplasmic catalytic domains acting at the peptidoglycan layer file:PSEPK/mrcA/mrcA-deep-research-falcon.md aPBPs operate in complexes bridging the inner and outer membranes via periplasm-spanning partners |
| GO:0003824 catalytic activity | IEA GO_REF:0000043 | REMOVE | Summary: This is a very general root-level term that provides minimal information. PBP1A is indeed a bifunctional enzyme with both glycosyltransferase and transpeptidase activities, but this annotation is too broad to be informative. Reason: This term is overly general and adds no value when more specific molecular function annotations (GO:0008955, GO:0009002) are present. The annotation to "catalytic activity" provides no meaningful information about what the enzyme actually does. |
| GO:0004180 carboxypeptidase activity | IEA GO_REF:0000043 | MODIFY | Summary: PBP1A possesses D,D-carboxypeptidase activity as part of its transpeptidase domain function. The enzyme can cleave the terminal D-Ala from the peptidoglycan pentapeptide stem. UniProt annotation confirms EC 3.4.16.4 (serine-type D-Ala-D-Ala carboxypeptidase). However, more specific terms exist. Reason: While technically correct, the annotation should use the more specific term GO:0009002 (serine-type D-Ala-D-Ala carboxypeptidase activity) which precisely describes the enzymatic activity. The general "carboxypeptidase activity" term is less informative. Proposed replacements: serine-type D-Ala-D-Ala carboxypeptidase activity Supporting Evidence: UniProt:Q88CU6 substituents of D-alanine.; EC=3.4.16.4 |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: PBP1A is correctly localized to the plasma membrane (inner membrane in Gram-negative bacteria). The protein has an N-terminal transmembrane helix (residues 7-30 per UniProt) that anchors it to the inner membrane. Reason: UniProt annotation confirms "Cell inner membrane; Single-pass type II membrane protein" with a defined transmembrane helix. The catalytic domains are periplasmic but the protein is membrane-anchored. Supporting Evidence: UniProt:Q88CU6 SUBCELLULAR LOCATION: Cell inner membrane UniProt:Q88CU6 Single-pass type II membrane protein |
| GO:0006508 proteolysis | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: PBP1A does have peptidase activity via its transpeptidase domain which cleaves the D-Ala-D-Ala bond during transpeptidation. However, the primary function is transpeptidation (forming new crosslinks) rather than simple proteolysis. Reason: The proteolytic cleavage of the terminal D-Ala is mechanistically coupled to transpeptidation - the acyl-enzyme intermediate is transferred to an acceptor amino group rather than being simply hydrolyzed. This annotation is not wrong but represents a secondary aspect of the core transpeptidase function. Supporting Evidence: file:PSEPK/mrcA/mrcA-deep-research-falcon.md Ser-type D,D-TPase activity (EC 3.4.16.4) that crosslinks peptide stems to the existing sacculus |
| GO:0008233 peptidase activity | IEA GO_REF:0000043 | MODIFY | Summary: Similar to the proteolysis annotation, this general term captures that PBP1A has peptidase activity. The transpeptidase domain acts as a serine-type D-Ala-D-Ala carboxypeptidase. Reason: This general term should be replaced with the more specific GO:0009002 (serine-type D-Ala-D-Ala carboxypeptidase activity) which accurately describes the substrate specificity and mechanism. Proposed replacements: serine-type D-Ala-D-Ala carboxypeptidase activity Supporting Evidence: UniProt:Q88CU6 Preferential cleavage: (Ac)2-L-Lys-D-Ala-|-D-Ala |
| GO:0008360 regulation of cell shape | IEA GO_REF:0000043 | ACCEPT | Summary: PBP1A contributes to maintaining cell shape through peptidoglycan synthesis. However, this term may be slightly overspecified - PBP1A participates in peptidoglycan synthesis which is essential for cell shape, but it is not primarily a "regulator" of cell shape. In Pseudomonas, PBP1A mutants show morphology/motility defects [Chen et al. 2017]. Reason: The UniProt keyword "Cell shape" maps to this term. The peptidoglycan layer synthesized by PBP1A directly determines bacterial cell morphology. In E. coli, PBP1B loss decreases cell stiffness [Pazos & Vollmer 2021]. While "maintenance of cell shape" might be more precise, this annotation captures the biological reality that PBP1A function affects cell morphology. Pseudomonas PBP1A disruptions specifically affect morphology [Chen et al. 2017]. Supporting Evidence: file:PSEPK/mrcA/mrcA-deep-research-falcon.md PBP1A disruptions affecting motility and morphology file:PSEPK/mrcA/mrcA-deep-research-falcon.md aPBPs provide mechanical stability (e.g., PBP1B loss decreases stiffness in E. coli) |
| GO:0008658 penicillin binding | IEA GO_REF:0000120 | ACCEPT | Summary: PBP1A is by definition a penicillin-binding protein. The transpeptidase active site serine is acylated by beta-lactam antibiotics including penicillins and cephalosporins. This is a core defining feature of the protein family. Reason: The protein name itself (Penicillin-binding protein 1A) indicates this function. The transpeptidase domain contains a serine that forms a covalent acyl-enzyme intermediate with beta-lactam antibiotics, inhibiting the enzyme. This is the mechanism by which beta-lactams exert their antibacterial effect. Supporting Evidence: file:PSEPK/mrcA/mrcA-deep-research-falcon.md The TPase active site is acylated by beta-lactam antibiotics UniProt:Q88CU6 RecName: Full=Penicillin-binding protein 1A |
| GO:0008955 peptidoglycan glycosyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: This is one of the two core enzymatic activities of PBP1A. The N-terminal glycosyltransferase domain (GT51 family) polymerizes glycan strands from lipid II precursors (EC 2.4.99.28). The domain is inhibited by moenomycin. Reason: This annotation accurately captures a primary molecular function of PBP1A. UniProt confirms EC 2.4.99.28 and the Pfam Transgly domain (PF00912). The glycosyltransferase activity polymerizes lipid II to form the glycan backbone of peptidoglycan. Supporting Evidence: UniProt:Q88CU6 ChEBI:CHEBI:78435; EC=2.4.99.28 UniProt:Q88CU6 Glycosyl transferase family 51 file:PSEPK/mrcA/mrcA-deep-research-falcon.md GTase activity (EC 2.4.99.28) that polymerizes glycan strands from the lipid II precursor |
| GO:0009002 serine-type D-Ala-D-Ala carboxypeptidase activity | IEA GO_REF:0000003 | ACCEPT | Summary: This is the second core enzymatic activity of PBP1A. The C-terminal transpeptidase domain acts as a serine-type D,D-transpeptidase/carboxypeptidase (EC 3.4.16.4). The enzyme cleaves the terminal D-Ala and transfers the acyl-enzyme intermediate to form crosslinks. Reason: This annotation accurately describes the transpeptidase domain function. UniProt confirms EC 3.4.16.4 and the Pfam Transpeptidase domain (PF00905). This is the enzymatic activity targeted by beta-lactam antibiotics. Supporting Evidence: UniProt:Q88CU6 substituents of D-alanine.; EC=3.4.16.4 UniProt:Q88CU6 Penicillin-binding protein transpeptidase UniProt:Q88CU6 Preferential cleavage: (Ac)2-L-Lys-D-Ala-|-D-Ala |
| GO:0009252 peptidoglycan biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: This is the central biological process in which PBP1A participates. As a bifunctional synthase, PBP1A directly catalyzes peptidoglycan biosynthesis by both polymerizing glycan chains and crosslinking peptide stems. Reason: This is the core biological process annotation for PBP1A. UniProt pathway annotation confirms "Cell wall biogenesis; peptidoglycan biosynthesis". The enzyme acts late in the peptidoglycan pathway, using lipid II to extend and crosslink the sacculus. Supporting Evidence: UniProt:Q88CU6 Cell wall biogenesis; peptidoglycan biosynthesis file:PSEPK/mrcA/mrcA-deep-research-falcon.md PBP1A acts late in the peptidoglycan pathway, using lipid II delivered to the periplasm to extend glycan chains and crosslink to the existing sacculus |
| GO:0016740 transferase activity | IEA GO_REF:0000043 | REMOVE | Summary: PBP1A has transferase activity via its glycosyltransferase domain, but this root-level term is too general to be informative. Reason: This term is overly general and redundant with the more specific GO:0008955 (peptidoglycan glycosyltransferase activity) annotation. Removing this uninformative annotation improves annotation quality. |
| GO:0016757 glycosyltransferase activity | IEA GO_REF:0000043 | MODIFY | Summary: PBP1A has glycosyltransferase activity but this term is less specific than GO:0008955 (peptidoglycan glycosyltransferase activity) which is already annotated. Reason: While correct, this should be replaced with the more specific GO:0008955 (peptidoglycan glycosyltransferase activity) which captures the substrate specificity. Since GO:0008955 is already annotated, this general term is redundant. Proposed replacements: peptidoglycan glycosyltransferase activity |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | REMOVE | Summary: PBP1A has hydrolase activity (carboxypeptidase function), but this root-level term is too general to be informative when more specific terms are available. Reason: This term is overly general and provides minimal information. The more specific GO:0009002 (serine-type D-Ala-D-Ala carboxypeptidase activity) annotation already captures the hydrolase function with appropriate specificity. |
| GO:0046677 response to antibiotic | IEA GO_REF:0000043 | ACCEPT | Summary: PBP1A is the direct molecular target of beta-lactam antibiotics. The transpeptidase active site serine is covalently acylated by penicillins and cephalosporins, inhibiting the enzyme and causing bacterial cell death. Reason: This annotation is legitimate and accurate. PBPs are the pharmacological targets of beta-lactam antibiotics, and their inhibition leads to bactericidal effects. The UniProt keyword "Antibiotic resistance" also supports this annotation. Mutations in PBPs can confer antibiotic resistance. Supporting Evidence: file:PSEPK/mrcA/mrcA-deep-research-falcon.md TPase inhibited by beta-lactam antibiotics (penicillins/cephalosporins) file:PSEPK/mrcA/mrcA-deep-research-falcon.md In Pseudomonas, PBPs are established beta-lactam targets UniProt:Q88CU6 Antibiotic resistance {ECO:0000256|ARBA:ARBA00023251} |
| GO:0071555 cell wall organization | IEA GO_REF:0000043 | MODIFY | Summary: PBP1A directly participates in cell wall organization by synthesizing and crosslinking peptidoglycan, the main structural component of the bacterial cell wall. The enzyme works in multi-protein complexes with hydrolases to coordinate safe insertion of new peptidoglycan. Reason: While cell wall organization is correct, the more specific term GO:0031504 (peptidoglycan-based cell wall organization) would be more appropriate for this Gram-negative bacterial protein that specifically acts on peptidoglycan. Alternatively, GO:0009273 (peptidoglycan-based cell wall biogenesis) could be used. Proposed replacements: peptidoglycan-based cell wall organization Supporting Evidence: file:PSEPK/mrcA/mrcA-deep-research-falcon.md participates in multi-enzyme complexes with hydrolases and divisome/elongasome components to insert and repair PG during elongation and septation UniProt:Q88CU6 Cell wall biogenesis/degradation {ECO:0000256|ARBA:ARBA00023316} |
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Download this section (compressed HTML)Q: What is the specific outer membrane lipoprotein activator for P. putida PBP1A - is it LpoA, LpoP, or another family member? E. coli uses LpoA for PBP1A and LpoB for PBP1B, while Pseudomonas employs LpoP, but the specific pairing with PBP1A in P. putida KT2440 is not established.
Q: Is P. putida mrcA (PBP1A) individually essential or redundant with mrcB (PBP1B)? In E. coli, either can be deleted but not both. In P. aeruginosa, most HMM PBPs are individually nonessential. Direct data for P. putida KT2440 is limited.
Experiment: Generate clean deletion mutants of mrcA and mrcB in P. putida KT2440 to test individual essentiality and redundancy. This would clarify whether PBP1A and PBP1B are functionally redundant in this organism as they are in E. coli.
Hypothesis: mrcA and mrcB are individually dispensable but show synthetic lethality when both are deleted, similar to E. coli.
Experiment: Characterize cell morphology and mechanical properties of mrcA mutants under various stress conditions. Based on E. coli data showing stress-dependent performance differences between PBP1A and PBP1B, similar studies in P. putida would inform functional partitioning.
Hypothesis: mrcA mutants will show altered cell shape or reduced mechanical stability under specific stress conditions such as high osmolarity.
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