The best host system is (3) Native protein in Baculovirus.

Option-wise explanation

• (1) GST fusion protein in E. coli

  • E. coli is prokaryotic and lacks N‑ and O‑linked glycosylation pathways.

  • GST tag aids solubility and purification but the expressed protein will be non‑glycosylated, unsuitable when proper glycosylation is needed.

• (2) His‑tagged protein in E. coli

  • Same limitation: His-tag only helps purification; E. coli still cannot perform complex eukaryotic glycosylation.

  • Good for large quantities of simple proteins, not glycoproteins.

• (3) Native protein in Baculovirus – correct

  • Baculovirus–insect cell systems (e.g., Sf9, High Five) are eukaryotic and contain ER and Golgi machinery capable of N‑linked glycosylation and some O‑glycosylation.

  • They routinely produce milligram quantities of properly folded, post‑translationally modified proteins suitable for X‑ray crystallography and cryo‑EM.

  • Glycosylation patterns may not be identical to mammalian, but are far closer than bacterial systems and are generally adequate for structural work.

• (4) Native protein in Pseudomonas fluorescens

  • P. fluorescens is also a bacterium; despite some engineered systems, standard strains do not provide complex eukaryotic glycosylation.

  • So it is not the best choice for glycosylated protein for structural studies.

Therefore, the host system that best matches “large amounts of glycosylated protein for structural studies” is native protein expressed using the baculovirus system (option 3).