starch utilization system complex

GO:0044570 · Other · Multiprotein complex · Proposed

A bacterial cell envelope-associated multiprotein system, which binds and degrades starch. [GO:0044570]

Synonyms: Sus complex

Part of: cell outer membrane (GO:0009279)

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Essential	Role
SusC TonB-dependent starch transporter	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	susC	ESSENTIAL	Outer-membrane TonB-dependent component that interacts with SusD and participates in starch binding and oligosaccharide uptake. `DOI:10.1074/jbc.R109.022848` Martens et al. 2009 review SusC as the TonB-dependent outer-membrane receptor of the B. thetaiotaomicron starch utilization system. `DOI:10.1128/JB.182.19.5365-5372.2000` Shipman et al. 2000 found that SusC contributes to starch binding and interacts with SusD in the outer membrane.
SusD starch-binding lipoprotein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	susD	ESSENTIAL	Surface-exposed outer-membrane lipoprotein that binds starch with SusC as a major starch-binding component. `DOI:10.1074/jbc.R109.022848` Martens et al. 2009 review SusD as one of the outer-membrane lipoproteins exposed by the B. thetaiotaomicron starch utilization system. `DOI:10.1128/JB.182.19.5365-5372.2000` Shipman et al. 2000 measured a major SusD contribution to cell-surface starch binding and provided evidence for an outer-membrane SusC-SusD interaction.
SusE starch-binding lipoprotein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	susE	UNKNOWN	Surface-exposed outer-membrane lipoprotein that enhances SusC- and SusD-dependent starch binding. `DOI:10.1074/jbc.R109.022848` Martens et al. 2009 review SusE as an outer-membrane Sus lipoprotein that is exposed to the external environment. `DOI:10.1128/JB.182.19.5365-5372.2000` Shipman et al. 2000 found that SusE contributes to starch binding and increases SusC-SusD binding activity in B. thetaiotaomicron.
SusF starch-binding lipoprotein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	susF	UNKNOWN	Surface-exposed outer-membrane lipoprotein associated with the starch-binding Sus complex. `DOI:10.1074/jbc.R109.022848` Martens et al. 2009 review SusF as one of the outer-membrane Sus lipoproteins exposed by the starch utilization system. `DOI:10.1128/JB.182.19.5365-5372.2000` Shipman et al. 2000 showed that SusF is surface exposed and protected from proteolysis by amylopectin, supporting its association with bound starch.
SusG alpha-amylase	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	susG	ESSENTIAL	Cell-surface outer-membrane alpha-amylase that hydrolyzes starch after cell-surface binding by SusC/D/E/F. `DOI:10.1074/jbc.R109.022848` Martens et al. 2009 review SusG as an outer-membrane Sus lipoprotein and model it as the surface glycoside hydrolase that degrades bound starch. `DOI:10.1128/JB.181.23.7206-7211.1999` Shipman et al. 1999 showed that SusG is an outer-membrane protein exposed on the cell surface whose main role is starch hydrolysis.

Taxonomic distribution

NCBITaxon:818 Bacteroides thetaiotaomicron — COMMON: The B. thetaiotaomicron starch utilization system is encoded by susRABCDEFG and includes the outer-membrane SusCDEFG constituents curated here. [DOI:10.1074/jbc.R109.022848]

Canonical examples

NCBITaxon:818 Bacteroides thetaiotaomicron — B. thetaiotaomicron is the model organism in which SusC, SusD, SusE, SusF, and SusG were characterized as outer-membrane starch utilization proteins. [DOI:10.1128/JB.182.19.5365-5372.2000]

Functions

cell-surface starch binding and degradation — Binds starch at the bacterial cell surface, positions it on SusC/D/E/F outer-membrane proteins, and hydrolyzes starch through the SusG surface alpha-amylase to initiate starch import.
- DOI:10.1128/JB.182.19.5365-5372.2000 Shipman et al. 2000 showed that SusC, SusD, and SusE cooperate in starch binding and that SusF is surface exposed in the B. thetaiotaomicron Sus system.
- DOI:10.1128/JB.181.23.7206-7211.1999 Shipman et al. 1999 established SusG as the Sus outer-membrane protein responsible for cell-surface starch hydrolysis.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral family groundings for SusC, SusD, SusE, SusF, and SusG in the starch utilization system complex. The initial record uses reviewed gene-family labels from B. thetaiotaomicron literature and avoids guessing InterPro, Pfam, NCBIfam, or UniProtKB identifiers that could broaden a Sus starch-system component to unrelated TonB transporters, SusD-like proteins, or alpha-amylases.
INTERPRETATION (RESOLVED): Should SusA, SusB, or SusR be curated as constituents of the outer-membrane starch utilization system complex? Martens et al. 2009 place SusA and SusB in the periplasm and describe SusR as an inner-membrane regulator, whereas the curated SusCDEFG constituents are outer-membrane proteins. SusA, SusB, and SusR are therefore out of scope for this outer-membrane complex.

Evidence

GO:0044570 GO:0044570 is an active cellular-component term for the starch utilization system complex, carries the exact synonym Sus complex, and defines the complex as a bacterial cell-envelope-associated multiprotein system that binds and degrades starch.
DOI:10.1074/jbc.R109.022848 Martens, Koropatkin, Smith and Gordon 2009 review the Bacteroidetes Sus-like paradigm and the B. thetaiotaomicron starch utilization system.
DOI:10.1128/JB.182.19.5365-5372.2000 Shipman, Berleman and Salyers 2000 characterized SusC, SusD, SusE, and SusF as outer-membrane proteins involved in binding starch to the B. thetaiotaomicron cell surface.
DOI:10.1128/JB.181.23.7206-7211.1999 Shipman, Cho, Siegel and Salyers 1999 physiologically characterized SusG as the outer-membrane starch-hydrolyzing component of the Sus system.

Curation history

2026-10-01T14:55:34Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-01T14:57:02Z codex · EXPANDED_RECORD — Added Sus synonym, parentage, outer-membrane SusCDEFG components, B. thetaiotaomicron taxonomy, starch-binding function, evidence, and component-boundary discussions.
2026-10-01T15:10:49Z codex · FIX_DISCUSSION_ANCHOR — Pointed the SusA/SusB/SusR boundary discussion at concrete Sus component anchors.
2026-10-01T15:23:48Z codex · ADDRESS_REVIEW — Addressed PR #1875 review issue #1876 by changing SusE and SusF essentiality from DISPENSABLE to UNKNOWN.

Source: data/structures/other/starch_utilization_system_complex.yaml