PSI-BLAST API (EBI)

PSI-BLAST API (EBI) performs iterative sequence similarity searches using position-specific scoring matrices (PSSMs) to detect distant homologs in protein and DNA databases.

Key Features:

• Position-Specific Scoring Matrix (PSSM): Constructs a PSSM from statistically significant alignments identified in initial BLAST searches and uses that matrix for subsequent database searches.
• Enhanced Sensitivity: Leverages the PSSM and iterative updating to detect weak but biologically relevant sequence similarities, enabling identification of distant evolutionary relationships and novel protein family members.
• Algorithmic Improvements: Incorporates a new criterion for extending word hits and a heuristic approach to generating gapped alignments, improving search efficiency and enabling approximately threefold speed increases over its predecessor while maintaining high sensitivity.
• Iterative Search Process: Refines the PSSM across multiple iterations by incorporating newly identified significant alignments to progressively improve detection of relevant sequences.

Scientific Applications:

• Protein family and superfamily detection: Identifies members of protein families and superfamilies that share weak but functionally important similarities, exemplified by applications to the BRCT superfamily.
• Evolutionary analysis and functional annotation: Detects distant homologs for evolutionary biology studies, supports functional annotation of proteins, and aids exploration of sequence diversity within large protein databases.

Methodology:

Begins with a standard BLAST search to identify initial similarities, constructs a PSSM from statistically significant alignments, applies the PSSM in subsequent database searches, and iteratively refines the matrix across iterations to increase sensitivity.