Cyclin/CDK complexes are serine/threonine protein kinases that act as positive regulators in cell cycle progression, and their overexpression or loss of endogenous inhibitors (e.g., p16, p27) are associated with various tumors, making CDKs new molecular targets for cancer chemotherapy. We established a budding yeast-based cell assay where induction of Xenopus cyclin A1 elevates CDK (Cdc28) kinase activity, leading to growth arrest; compounds rescuing this arrest are potential antitumor candidates targeting cyclin/CDK-mediated cell cycle regulation. Through microbial screening, we identified a novel metabolite belactosin A from Streptomyces sp. KYI1780 (isolated from Kanagawa soil). Isolation involved fermentation (28°C, 48h, 30-liter jars with sucrose-yeast medium), column chromatography (Diaion HP-20, ODS-AM, silica gel) and chemical modification (Boc protection/deprotection). Belactosin A has unique physicochemical properties: highly water-soluble (>10 mg/ml), molecular formula C₁₇H₂₇N₃O₆, structure containing a novel amino acid 3-(2-aminocyclopropyl)-alanine (AcpAla) and a β-lactone. It showed no antimicrobial activity against Gram-negative/positive bacteria up to 0.1 mg/ml. In vitro, belactosin A exhibited antiproliferative activity against HeLa S3 cells with IC₅₀ 51 μM (low potency due to poor cell permeability; ester modification enhanced activity). Flow cytometric analysis revealed it arrested HeLa S3 cell cycle at G₂/M phase in a dose-dependent manner (decreased G₁, increased G₂/M after 24h exposure). Thus, belactosin A is a novel natural product that restores cyclin A1-induced yeast growth arrest and inhibits human tumor cell cycle progression at G₂/M phase.